I bought a kit for my Macbook (same ram I believe) from OWC and have had no problems with. Price was the best I could find that didn't seem shady. It was $125 for 3GB at the time, but they've come down significantly since then.
Another vote for OWC - macsales.com - but Crucial is always a good bet. I have also use Patriot RAM that I bought from New Egg and never had an issue with it wither.