I haven't ordered cups, but I've ordered quite a lot of other bra-making/lingerie from (Canadian) Bra-makers Supply. I know they have cups and I've found their selection and quality to be excellent.
http://www.bramakers.com

Thanks guys for all the info.

My daughter-in-law just bought a set for $8 in Nordstrom's lingerie department. She wanted them to insert into a nursing bra that didn't have much "cover." The sales lady said most people buy them to get extra push-up.
Carol