The unfortunate thing about women's sizes is that they vary greatly between manufacturers, so you have to guess by their individual size charts (which have links right on the lingerie page) - like some would list you (by your waist size) as an extra-large but others would list you as a medium.
Having worked in a thrift store where some of the local cross-dressers would try to find their sizes, I can say that it might help doing the same in trying to figure out what cuts and sizes work for you that way. Don't go to any of the church-run shops for obvious reasons, and they just don't have decent clothes anyway. Dressing rooms are unisex, so you won't have any trouble taking anything in to try on.
I hope this helps.