What reputable stores exist for purchasing RAM for a Mac?
Personally I go with OWC. They are more famous for all Mac related upgrades. Here is the memory upgrade link http://eshop.macsales.com/shop/apple/memory/
Main advantage is you can choose RAM based on your model/ make of the year etc...
Once you figure out what brand(s) of memory you're looking for, the place to start shopping is DealRAM.com.
You put in what computer you have, and it'll tell you what your options are and where to buy at the best price.
I'm surprised no one has mentioned RamJet for Apple memory. Their prices are good, shipping is excellent and their web site is optimized for eliminating mistakes in ordering the incorrect memory.
I find their site to be the best reference on the web for rapidly finding the memory needs for any Mac bar none - even Apple's on-line documentation pales in comparison to RamJet for memory specifications or planning an upgrade.
They use pictures of the products and have very good descriptions to figure out exactly what mac you own. I've made perhaps 8 purchases from them and shipping, returns and customer support are top notch. I do regret buying from one of the "cheapest" vendors linked above when the product they sold didn't work and they charged me incorrectly for the return / exchange. It eventually got sorted, but I'll never get back the time and effort to clear up their mistakes on that one order.
Do your homework on what happens if you need support down the road and know the time periods for returns / warranty / support if you care. I'm more value sensitive than price sensitive but RamJet is generally very competitive on price as well. I'd also put Crucial and OWC in the tried and true category, but I've never seen a time when their prices were lower enough for me to not spend my time and $$ with RamJet.