Andrew, the only type of focal reducer you'll be able to use on your dob is a 0.5X reducer. These come in both 1.25" & 2". Andrews carries both, the 1.25" being $29. Look under their Guan Sheng banner:
http://andrewscom.com.au/site-section-10.htm
This is the reducer I use with my astro video camera. Very effective.
Just note that you could run into focusing problems though. You may find that the draw tube needs to be wound out further than it can go. If that's the case just use an extension tube.
Derek mentions the reducer from Myastroshop. If I'm not mistaken it's the same item as the GSO one, just rebadged. Excellent item too.
Which astro video camera do you have?
Mental.