I used a combination of discount codes and joining a cashback website, believe it or not but Crucial ended up being the cheapest! I started a thread here http://forums.macrumors.com/showthread.php?t=228073 that explains what I did.... I'm not sure when Crucial will end their 5% sale though but if it has finished then you should still be able to save something like 12% of the list price.
I did find a few places that stock Kingston RAM but the prices are all over the place so it would involve a fair bit of searching around and off the top of my head I can't remember where the cheapest place was.
You could also try Offtek they seem to sell RAM that is priced similar to Crucial...(they also have a 7% cashback deal on Quidco)
Oh...in case you're wondering...I do like Quidco!!!
If you want crucial memory (which quite a few people regard as the best, myself included) then go to dabs.com and buy it cheaper there than from crucial themselves. You may pay a bit more but they have an excellent reputation for making solid error free RAM.