I found next to nothing on Craigslist as well, but plenty on Facebook marketplace. One thing you want to keep in mind, depending on how much work you want to do and what kind of tools you have is whether to go with a sealed drum vs one with a removable lid. With a sealed drum, you'll need to cut the top off, which is not desirable in my opinion as there are issues creating a tight seal between the drum and the lid. Better to go with a drum with a removable lid.
Second, if you can find a drum that is unlined, even if it costs more, it will cut back significantly on prep work as you won't have to remove the synthetic liner with burning, then cleaning out with a wire wheel.
Last, I would avoid drums with ridges like the photo below. The lower set of ridges offer no smooth/flat surface in the area you'll want to mount your lower exhaust inlets.
At some point a few months ago, I decided to go with a drum identical to the one used in this thread by sudsandswine below:
https://www.bbq-brethren.com/forum/showthread.php?t=281554
I liked the smooth surface of the drum he used, as well as the fact the lid had no bungs. After a couple of weeks of searching, I found one in IN, about a 2.5 hour drive from us for $20 and picked it up a few days later. It had everything I was looking for - no liner, no ridges, removable lid, and the lid is flat with no bungs. Once I get the holes drilled for grate hardware, exhaust inlets etc, I'm going to have it sandblasted, then it gets a nice coat of paint.