Have you tried a professional bra fitting? Some lingerie stores have them... I've also found the saleswomen at Nordstroms (if you have one near you) to be fantastic as far as helping with getting the fit right and they carry bigger sizes in the stores.

You should be able to find a supportive bra that doesn't dig in to you with the underwire (or doesn't have an underwire - I had a sports bra with no underwire, and it was fine, despite my size being on the larger side... think ~5D!).

If your straps are constantly falling, it could be that you don't have a small enough/tight enough band. It sounds weird, and I don't understand the mechanics, but I've found that to be true to me. If you have room to tighten up the band, you might give that a try, though you might need one that's a little smaller around your ribs. (I think that when the band is properly tight, it provides some support so that your straps are not doing all of the work?)

In a pinch, I've also put a little moisturizer on my shoulders to make them less dry, which seems to help (a little?) with keeping the bands from slipping.

Sorry... you probably weren't looking for advice. I'm on the bigger side, and I'm much more comfortable (most of the time) with a (well-fitting) bra rather than without.

They are definitely too insanely expensive though. And, not enough stores carry bigger sizes. Heck, not enough brands make bigger sizes. I was wearing the wrong size in a really crappy bra (that looked terrible on me, apparently) for YEARS before some friends dragged me to the lingerie store as part of a makeover to get a proper fitting and buy some decent bras, and it was eye-opening. It was a bit weird, at first, having the store people help with something so intimate, but it was really helpful to figure out what I needed.

I can't speak to styles though, sorry! I'm pretty horrible with style/fashion type stuff. I think the ones I've got are nice enough looking, but I'm probably a bad judge!

Sorry you're having so much trouble with them! It really sucks that so many of them don't bother with bigger sizes, and those that due are usually imported and pricey. My bras are generally the most expensive thing I'm wearing, and actually cost more than my shoes! How crazy is that?!?!

Good luck!