I use Enom for .org's although I've also got an account at Domainsite that I don't use any more. I think Enom's transfer-in process is very user friendly.
Rather than paying full Enom prices for your domains you'll want to find yourself a discounted account with them though, I've got one $8.49 account and a $6.95 account for bulk buys (over $100). You'll be able to find someone to sell you one on DNForum.com or Ebay.