What ram to buy

I just bought a new 27" I5 Imac and would like to upgrade the ram to alteast a 8 I know it can be bought for alot cheaper then what apple does where do I buy it from?

I got mine from OtherWorldComputing. There are numerous other places you can get it from, but these guys have a solid rep and decent prices.

I literally no more then 20 minutes ago just upgraded my 2011 27-inch iMac to 12GBs. Before that I upgraded my Macbook Pro as well. In each case, I've used memory from Crucial RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, SSD at Crucial.com and they've exceeded my expectations. Fast performance and good pricing. I'd done a lot of research before I started using them and it seemed Crucial had the very best user reviews with regard to performance, reliability, and pricing.

I'd go with them.