+1 for metal supermarkets, superior metal centre (SMC) also is starting to make inroads, depending on where you are.
Try hardware stores, or big chain hardware store, like Home Depot, Rona or Lowe's. My local ones typically have some stuff available, albeit way overpriced. I've also seen aluminium panels in places like Canadian Tire, in the automotive repair section.
I know that locally, I've had some success in buying plastic offcuts from a signmaker. I'm talking about a company that specialises in signage and logos for retail stores. They typically cut out patterns and have a lot of leftovers. I just walked into the office and asked.
However, the cheapest source of aluminium I know are old streetsigns and construction warning panels...