In most Western countries - and certainly in the U.S. and Britain - adult bridesmaids pay for their own dresses. If the adult bridesmaids are still financially dependent on their parents, as college students often are, their parents do as your parents did, and pick up the bill. Parents of child attendants similarly buy the outfits for the children.
There are countries where this rule is not followed -- South Africa comes to mind -- but if you lived in such a country, someone would have offered to buy you a bridesmaid dress by now! If your problem bridesmaid comes from another country, or is part of a close-knit, traditional family of foreign origins, it's possible that she is only aware of her own people's customs. Try explaining tactfully that the local custom is different from what she's used to, and that you had no intention to offend.
There are, of course, other reasons a bridesmaid could balk. First, she may regret agreeing to be a bridesmaid, so refusing to pay for her own dress is a convenient reason to resign. If she is a distant acquaintance chosen so that your bridesmaids would match a specific number of ushers, a relative included as a duty, or a formerly close friend that hasn't seemed so close in recent years, there's a very good chance that she just doesn't want to participate. Her absence from the wedding party will be no loss, and no one will think twice about any uneven numbers that may result.
Second, she may be unable to afford the dress, but embarrassed to say so. Bridesmaid dresses run about double the cost of a comparable evening gown. If you're accustomed to wearing well-made clothes in fashionable cuts and subtle colors, you may gravitate to bridesmaid dresses that have a $300 price tag. That's chump change compared to the cost of a formal wedding gown, but it's much of a month's rent to a bridesmaid who is just out of college, trying to pay college loans out of her tiny first-job paycheck, and unwilling to ask her parents for help.
Third, she may hate the dress you've chosen. Even best friends ordinarily do not dress alike, which should give us all a clue about the difficulty of choosing an expensive dress that will please three to six women who may or may not be friends. If your problem bridesmaid's figure is the least bit unusual, the dress may hate her too.
There are three approaches to the problem of squeezing your bridesmaids into their dresses without damaging their wallets or their egos.
The most traditional approach, approved by etiquette gurus of the 1960s, is that the bride chooses the bridesmaid dresses, and the bridesmaids grit their teeth and wear them without a murmur of protest.
It's important to note that this rule dates from an era when most couples married very young. According to From Front Porch to Back Seat, a scholarly book on twentieth-century courtship practices, marriage the summer after high school graduation was the ideal, even for youngsters who were bound for college. If the bridesmaids were friends of the bride, they were 17- or 18-year- olds whose parents picked up the bills for the wedding outfits. And they truly might wear the dress again at a college formal.
Today's brides are usually post-college rather than just out of high school, with friends who are also in the first years of trying to be financially stable and independent. So there's a much greater need to fit the dress to the financial means and personal tastes of the wearers. And the "shut up and wear it" approach always had the downside of limiting your pool of potential bridesmaids to the women who look good wearing anything and the ones with enough sense of humor to not care if they looked awful. (The humorous ones resurrected their bridesmaid dresses as Halloween costumes or entered them in Ugly Bridesmaid Dress contests.)
A second approach, if you want a formal look with a low price tag, is to treat the bridesmaid dresses as essentially disposable, and therefore buy the cheapest ones that the 'maids will agree to wear.
Like theatrical costumes, bridesmaid dresses are mostly seen from a distance, so shoddy workmanship won't show. Unlike theatrical costumes, bridesmaid dresses don't have to stand up to vigorous dancing and singing (we hope!), so it's unlikely that the dresses will fall apart mid-wedding. (Though this is a story in Bridal Bargains about a maid-of-honor's dress that did come apart at the seams during the ceremony!) If your reception calls for dancing until dawn, it might be wise to encourage bridesmaids wearing little cheapies to bring a second outfit (and they may feel prettier, too).
Given a lucky day at the J.C. Penney sales rack, a traditional bridesmaid dress can cost as little as $59. If you broaden your search a bit, to include the regular evening gown section, the "better dresses" department (for a daytime wedding), or the Chadwick's catalog, you can find remarkable bargains. And some of these dresses are very nice, and can be worn again unless your bridesmaids' entire social lives revolve around the better casinos as Monte Carlo. The "clothing and accessories" section on Ron & Debbie's Penny Pinchin' Wedding Page has even more ideas for controlling dress costs.
Once you're in the regular department store dress sections, it's only a small step from looking for inexpensive "disposable" dresses to considering dresses that the bridesmaids genuinely might wear again.
It has become conventional, in many circles, for bridesmaids to wear long formal gowns even for a late-morning or early afternoon wedding. (Etiquette experts don't approve the practice, but popular customs change more quickly than formal etiquette.) While this practice makes a pretty picture, it complicates efforts to choose an acceptable dress.
Back when matching bridesmaids were invented, in the mid-nineteenth century, the bridesmaids were dressed appropriately for the time of day. The long, fancy dress worn at an afternoon wedding was nothing more than the sort of dress that a young woman might wear to any major afternoon party. In today's terms, it would be the same as the dress you might wear to a garden party, an art gallery opening, or a wedding where you were a guest!
Unless the bride is wearing the most elaborately formal dress possible, it's worth thinking about going back to the old rule and dressing bridesmaids in clothes that women wear for "normal" dress-up occasions. While this means evening gowns for a formal evening wedding, it may mean elegant street-length dresses or suits for an afternoon wedding.
Exactly matching dresses are optional: many stores will stock slightly different styles in the exact same shades of sage green, aubergine, aqua, or other fashionable colors. It's also very elegant, stylish, and traditional to have each bridesmaid choose a white dress appropriate for the time of day. (Different shades of white blend together just fine, and the bride's more elaborate dress, veil, and dazed expression of joy distinguish her from her attendants.)