Not speaking from experience but from knowledge of where folks up here have found drums; check in your area for any local bakeries, soup processors, or any other food service or distributors who might get their products in bulk. Keep an eye on craigslist for your area, sometimes they are listed on there also.
You might want to check the yellow pages also, or google 55 gallon drums + your city name and see if anything pops up.
Your best bet though is to find some type of food service, bakery, restaurant, or food processing plant and give them a call and see if they have any.
Good Luck, sorry I couldn't be more help.
Treasurer, Great Lakes BBQAssociation
Rubbed, Smoked, and Sauced Competition BBQ Team-
22 1/2" WSM, 2 18 1/2" WSM's, 22 1/2" Weber Kettle