otherworldcomputing.com has some great ram, I havn't had any problems with ram from them. I've bought quite a few sticks from them for various machines.
Other than that Crucial is my other supplyer. I havn't had anything but good luck from them either.
I've been getting my RAM from Coast Memory for years. The RAM is good quality and has never let me down. But, if you want the best RAM, get it from Crucial
RAM Seeker lists deals for all types of macs. Check out the site RAM Seeker for some more information. I bought a gig of RAM for my PowerBook from a site listed on RAM Seeker. I've had no problems.