Years ago, I read an article about the American fear of large cup sizes. In essence, not only were large cups with a small band hard to find, but cup sizes weren't accurately labeled because, supposedly, some women are freaked out by the idea of having wearing a large cup size. According to the article, Europe doesn't have this problem, at least not to this degree. I see that you are Canadian, so I don't know if the "fear" mentioned above applies in your neck of the woods. In any case, I can't find the article now. That blog may not exist anymore.
However...while searching for it, I found lots of articles about vanity sizing for bras, and about how bra sizing changed recently but the general public isn't aware of it...until they go in to buy a new bra. I didn't read all the articles because I'm at work, and they may interpret a pic of a woman in a bra as inappropriate around here. Anyway, if you are interested, do a search with terms like "bra cup size fear" and see what comes up. It's pretty interesting.
Lastly, if you can't find a bra you like with convertible straps (to get that X or racerback look in order to minimize slippage), I suggest looking around for one of those things that will connect your shoulder straps in back. Something like this. http://www.amazon.com/2001-Fas.....B000144M54
In a pinch, I've "macgyvered" my own solution using a bit of ribbon or elastic. I just loop it through the straps in back and tie it. I will sometimes add the strap first, then pull the bra over my head to get it on. Or I'll ask a friend to help me if I can't reach back that far. The thingie I mentioned above, or this make shift suggestion, would be a cost saving measure until your weight and bra size is stable again.