I've been lurking here for a while getting ideas. Today I'm finally putting the lid on my UDS for the first time!
Here is what I've done that others may find helpful: My drum was one I found that had been used >20 years ago for burning trash. It was nothing but rust inside, the liner had been burned out through years of use. I used electrolysis to remove the rust. I took iron rebar, attached it to the positive terminal of a battery charger (6amp), and suspended it in the barrel. I attached the negative terminal to the drum (sand it a bit for a good connection). I filled the barrel with water and plugged in the charger. 24 hours later, rust was peeling off the barrel and the 3/8 iron rod was an inch thick with rust. I didn't have to do any sanding to the inside (although I did, and I cleaned it with TSP and rinsed several times). After it dried I coated it with oil to prevent new rust. I use a bug sprayer filled with vegetable oil to season my cookers.
The lid was hard to find. I learned that the companies who build small storage sheds get their paint in these drums and the one in my area was eager to give me as many lids, locking rings and drums that I wanted for free. They are lined. It has taken me a week of working 2 hours a night to remove the liner from the lid.
Anyway, that was how I got my drum and lid. I haven't seen these resources posted so I thought I would share. If you can't find an unlined drum, you can find an old, rusty drum and use the electrolysis method. It makes life much easier than trying to get that liner out.