I always had the slipping problem too, until I was properly fitted. I went to a shop that specialized in undergarments and prosthesis. The gal that ran the shop even had a sewing machine in the shop for alterations and quick fixes etc. I will say, after I was fitted, my band size went down two sizes, and my cup size increased five sizes. WAY better fit! I'd been compensating (obviously) by buying a larger chest size to accomodate the lack of fabric in the cups. The regular stores just didn't (and still don't) carry my size, so I pay a bit more, but have fairly comfortable bras. She also has me in the computer, so I can even send hubby in to get new bras, and he comes home with the right ones!
I recently took my daughter into Penney's to be fitted-figured the trained salesperson should know more than mom-and she measured her three cup sizes UNDER what she needed to be. She could never have squeezed into what she'd been measured. I guess I'm trying to say, if you have a specialty shop nearby, I'd try them over a chain store. I've never tried VS-kind of doubted they'd have my size, and Macy's said special order only-maybe, so I just visit my local specialty store! She's a sweet gal, wonderful service, and I'm happy to help keep her in business.