Your best bet is to buy them online. Either thur ebay or a home brewstore.
Unless you're content to wait for a deal. But the soda industry doesn't use them any more and scrap metal dealers like them for the stainless steel. Also more and more home brewers are realizing they make excellent secondary fermenters. So I look to see the supply dwindling or the price increasing over time or both.
Do what you like!
Brew what you like!