Online search or yellowpages. Food grade is the best if you are going with used (Rare find for me), but you could actually find one that was used to ship dry componets that should work also. There are different gauges of material used in their construction as well, usually the barrels with the ring clamps on the lids are the heavier gauge and these are also the ones that were used for dry shipping. Paint drums are a good find in my opinion, atleast I kind of sort of know what was in them.
Personally, I think that after a good cleaning to get the residue out, sanding/wire brushing to get the rust out and a good burn in fire cleansing, you are good to go. The sad part is that some of the stuff that was in those drums is very bad stuff. VERY BAD STUFF! Unless you know what was in the drum and are comfortable with it, lean towards a different drum.