I always buy RAM for my Macs from http://crucial.com/. I have never had any problems.
NewEgg – cheap and reliable
I use crucial.com too. Great answer @johnpowell. :-)
I should add that you can sometimes get a better deal at Crucial if you go through http://dealnews.com/memory/ and use the link to Crucial. Last time I bought RAM I saved about 25% using the link at Dealnews.