If you know someone who works at a bar or restaurant that uses beer kegs, talk to them. They may be able to "sell" them to you for the price of their deposit. Otherwise they need to be returned so they can get their deposit back.
Giving or selling used kegs can be a liability for businesses because they may contain beer left over in them. It helps if you have a good relationship with an owner.
Otherwise you can find them on ebay but I think you'll pay more.