Lots of places online... newegg... maybe outpost.com.
Alternately, find a local computer store---a real one that caters to businesses, not some "computer store" like CompUSA or similar big box stores.... They'll either have them or will known someone who does.
Do a search on froogle... or better yet, check out pricewatch.com and find specific part numbers, then search for those on froogle to try for a better price.