Bra sizes...HELP.

Right now I'm breastfeeding so I won't need this info until I wean my son, but I have fairly average-sized breasts (about a B-C cup) but a very small ribcage. My bras always fit my boobs well but are too big around, even on the smallest setting. If I shrug my shoulders it slides up a bit and it doesn't provide good support because it's too loose. I've never found a bra that fits well all around. How do I find that? I really don't want to spend a lot of money, just need some simple bras that aren't too big around for once 😅