It wasn't really that hard to buy Mac RAM before. The RAM has been standardized for years, although there are rare incompatibilities. You can buy from anywhere, and there are price comparisons here for example: http://www.dealram.com/ (They also do pricing for flash media cards, etc.)

I tend to stick with vendors I know are Mac-centric, because at least if I have a problem I know they'll know what I'm talking about, and I know they will not say "Oh, we don't guarantee it'll work in that". My vendor of choice for a few years has been Data Memory Systems in New Hampshire. http://www.datamem.com/ May not be absolute lowest price, but they're always close enough for me. More importantly, I have called them once about a bad memory module. They answered the phone quickly, accepted responsibility quickly and overnighted a replacement. Memory doesn't fail too often, but when it does, you want it fixed pronto, and I can attest that these folks are quick to make it right.