People have been asking where to get drums, in several of the competition I went to. After you read this you should spread this around.
Really anything circular can be turned into a drum. I have seen hobbyweights using the Aluminum bottles, or MWs or HWs using gas tanks. To add bumps to your drums. For smaller drums you could put studs and screws. Bigger bots have more variety, so be creative.
I hope this helps you on the journey of building a drum bot.