I haven't been to a party like that, but I did build-my-own purse at a store in Philadelphia once (it was called Viv Pickle). It was not cheap --- at least $50 for a tiny 5'x10'x2' purse with only the basic fabrics .... and you had to pay more for fancy fabric and pockets, etc...
But I think it would be pretty fun, if you bough some basic purses or tote bags and had all kinds of things for the girls to decorate them with. Sequins, and beads, and fabric paint... that kind of stuff. I've always been pretty crafty, so I would have loved that when I was 13 (OK, I would still love it!) I've seen basic tote bags pretty cheap at grocery stores for the bring-your-own bag. I would have a good walk around Jo-Ann or Michaels. I know our Jo-Ann has classes for kids too, and they do birthday parties. You might want to look into that. Or paint your own pottery place. We have one in my town, and it is so much fun and not too expensive. I actually threw a paint your own pottery bachelorette party once, and everyone really enjoyed it.