From my experience, Ebay is the best place to find your RAM if you know what you need. No iMac experience here, but I've had a lot of luck on my old G4s on Ebay and couldn't find anything better even near the prices there for most upgrades. Obviously look at the feedback and look at the actual description on what you are buying. I've also sold some RAM on there myself and always try to list exactly what I know it will work on, because I've tried it.
I've yet to have any major issues on anything I purchased or anything I sold - but I was very careful to look at what I needed and was buying and what how I described what I sold. Only problem I had recently was on a video card and I was able to fix that (poor application of thermal paste) and I probably could have gotten a refund if I really couldn't fix it myself as the seller was responsive and nice.
You could try OWC or Crucial as others have suggested or Newegg or many other sites, but, on older items, it's usually hard to find a better deal than Ebay as long as you know what you need and research the item and seller and have some patience for a good deal to come up.
