Yikes, that's a tough one! Do they sell backplates and wings?
Surely, if they sell NOTHING that fits you, they can't *make* you buy something that doesn't fit/feel comfortable? You could tell them that could be unsafe...
I'm sure they couldn't ethically suggest that a CUSTOMER buy something that doesn't fit.
You should go to other stores and try on BC's. There must be something out there that would fit you...
When you find the right one, perhaps the store you work at could order it in for you.
Since I'm petite and have curves, I wear a Genesis Athena, and love it. It fits me properly and is weight integrated, has padding, a depth compensating cummerbund, and has some pockets and D-rings.
Why don't you help us out by telling us your body shape - someone out there might have the perfect suggestion for you...