Hmmmm ... you really must have a hard time! I can give you answers/suggestions for most but necessarily all of your requirements.
In my search to find the right bra I've searched high and low and everywhere in between. At my normal weight I was technically a 32D, but that's next to impossible to find. (You think 32B/C is hard to find, without padding, try a 32D, almost impossible. Now try to find one without padding, there's more than you'd think!) Anyway my go-to bra is from Victoria's Secret, I-Pex Demi. I'd get a 34D and I'd keep my bra on the tightest setting. But before I get ahead of myself, make sure you're the size you think you are! I know everyone says that but really if you're not the size you think you are you'll run in to even more trouble!
Okay, now if that VS bra doesn't work for you my next suggestion would be to try higher end department stores. My next best bra shopping was at Nordstrom. I found a 32D strapless bra there!
It's not something you can easily find! Needless to say it
was not!!! cheap, but then again it was the only time I've found a strapless that fits me. So I'd suggest going to Nordstrom, Macy's, etc. where they'd have a nice sized lingerie department and access to a good bra specialist!
Unfortunately I have the opposite problem than yours, grad school made me gain weight and I'm now 15 pounds off from my normal 115.
So I had to go up a size until I can get back to there. Which reminds me, even though you're not where you want to be (as in you'll gain some weight and things will change) get something that fits you NOW! You can't predict the future, you can get a larger bra when you are larger!
P.S. Oh and as far as the stretch thing goes. My understanding is that by their nature, bras need to be 'retired' after a few months to a year or so. They natural adjust with so much wear and tear. I hate that, but I've realized it's true. Then again, while my bras stretch, it takes a very long time for them to get to a stretched out point that I notice. I notice the underwire moving much sooner! So if you're having stretch problems within a month or two maybe you're not in the right bra to begin with!?