Apple offers one: Apple USB SuperDrive - Apple Store (U.S.). However, it's a slot drive and I prefer a drawer drive.
The one drive that I've been looking at is from OtherWorldComputing.com. If you want a portable drive this might suit your needs: OWC MRSSBDR6X Slim USB2 Portable Blu-Ray 6X Burner... in stock at OWC..
A non portable (with its own power supply) might be this one: OWC Mercury Pro 800/400+USB2+eSATA SuperMulti-Drive up to 24X DVD, 12X DL DVD, 48X CDRW which could connect with USB or Firewire if you have a Thunderbolt to Firewire adapter. I use one with an external HD.
I've had good results with EHD from OWC but haven't purdchased an external optical drive as yet. So look around. This may be a good place to start.