Hi and be welcome to the forum!
Actually you can buy any DVD burner and put it in a USB2.0 case. This is often cheaper than buying a factory-assembled solution and it gives you a wider variety of choice.
Depending on your needs, there are quite a few DVD burners that can be considered to be very good. But again, it depends on your needs. I think that a visit to our Recording hardware forum (mind the brand-related subforums) would do you some good. Read about the burners, their capabilities etc. Also, check the Reviews section on the frontpage for CD Freaks reviews.
I hope this helps a little. If not, just ask! Or, use the search functionality to find your info!