Why not ask your closest girlfriends to be your bridesmaids with a handwritten note? You can surprise them by mailing each one a card or you can hand deliver the cards when you're out together.
This card, by Jenny Sweeney Designs is available online. You can also find cards for maid-of-honors and flower girls. And once the wedding is over, don't forget to send a special note thanking your bridal party for being such an important part of the wedding.