I was going to do before I moved from Seattle to Denver and had to get rid of my barrels. A couple of things:
1. Make sure that the barrels are food grade. Like the kind honey comes in to processors. For the insides are cover in a plastic coating to keep the food stuff safe. They are plentiful and cheap. $15 a piece.
2. Stainless would be the better choice for the rust reason. The only down side of SS is the cost. IF you can find one that is inexpensive please let me know because I would love to brew 40 gallons at a time!
Are you looking to get 55 gallon drums or something smaller? Because if you are looking to just do 20 gallon batches you could get someone to cut two kegs and weld them together as one 30 gallon kettle. For probably less then a 55 gallon SS drum
Hope this helps,