In a way I'm kind of lucky. While I am not out to her and she has been extremely negative about cross-dressing and gender-fluidity no matter how I've brought it up, she is ironically the same size as I am. Her chest measurement (though not cup size, of course!), hips, and skirt-length are all quite compatible. So I've been able to try on her clothes to at least get a sense of what "feels" good, what might look good, etc. We have very different coloration--I'm very sallow, while she is very ruddy--but it's made things a little simpler, anyway.
I'm having good luck online, which is great, although I'm constantly in dread of the "What's in that box you got today?" question from her or the kids. Usually I'm the one to deal with the mail--finally a plus in having lazy family members! And soon I think I'll know enough to go the thrift store route. I'm definitely finding that I very much like very feminine lingerie--g-strings, high-cut panties, lacy bras, and the like. They make me feel very feminine, which I love. Externally, long skirts and blousy tops seem to work, though I really need better breast forms to get the real effect.
Thank you folks for your help. 🙂