I got my barrel from a local reconditioner for $45, with the coatings all removed. I recommend it as an easy way to do it. You might get a coated one cheaper than I did or even free, but then you've got to deal with cleaning it up.
Try a place like this: http://milfordbarrel.com/
If you're not near Milford, google "steel drum reconditioner" for your location.
Tell them you want it uncoated/unpainted, just a bare drum. Call ahead, they may need time to get it ready for you (pull it out of the pipeline and set it aside).
Since you're talking about making a UDS, tell them you want an OPEN HEAD drum, and you want the lid WITH a bung hole and NO LID GASKET. This will all make your life easier.
Let us know how it goes.
<edit>Sorry for the weird caps, but they were very confused when I told them I wanted an open head drum for a smoker, they expected I'd want a closed head to make a horizontal smoker. Just be clear about what you want.