I paid for their dresses, and they are paying for alterations - plus, I'm paying for hair and makeup. I'm in a position where I can afford to do it, and they are doing me a great honor by coming all the way to Ireland for my wedding.
On the other hand, it's totally acceptable and rather common not to do it, too. Considering the expense of weddings and how tight finances can be, I'm sure good friends would understand about having to buy their own dresses. I know I've bought quite a few of my own bridesmaid dresses in the past!