OWC (MacSales.com), DMS (datamem.com), and Crucial are the three most obvious recommendations, plus whatever you can find on NewEgg.
Of those, I now avoid Crucial, as their prices have crept upward and they have some pricing policies I don't approve of. OWC and DMS will both sell you RAM for your exact Mac and will guarantee that it'll work in it, have lifetime warranties, and very reasonable prices.
Go to the site of either and you can confirm (though I can pretty much guarantee it's fine) that the config you're interested in will work. You can save a couple bucks by going with NewEgg, but given the oddball problems flaky RAM can cause, to me it's worth the extra $5-10 for the peace of mind, if nothing else.
I can also guarantee that DMS is VERY quick and fair with warranty replacement the couple of times I've had a stick fail at work (both infant failures); haven't yet had any OWC stuff go bad on me, and I've started leaning toward them mostly because of the company's green policies behind the scenes.