Antique Store
Antique stores are fantastic places to find anything considered "old." They are a good place to start your search for old milk cans. If there is farm land nearby, head out to an antique store near that area. It's more likely farmers sold milk cans to these stores. If you can't easily find a can, ask the sales associate--who typically is the owner of the store--if he has any older milk cans. He should know if one has come through or not.
Dairy Farm
Consider going straight to the source: dairy farms. These farms may still have some old equipment just sitting around taking up space and collecting dust. Stop by one of these farms and ask the owner if she has any she is willing to give away or sell. Cost will vary as there is no set appraisal value for old milk cans.
Ebay
Ebay often has old milk cans for sale daily, with many of them differing in size and design. Some are plain old aluminum cans, while others have a farm's logo on the side.