Here in Australia, I buy from scrap metal dealers located in areas where there are a lot of machine shops, they sell there bar ends to the scrap dealer, mainly brass rounds and aluminium plate.
Aluminium retailers often sell offcuts at reduced prices. If you are prepared to hold a bit of steel stock, the main steel suppliers are good, but you will have to pay a minimum price. My local has a minimum of $50, but that can be made up of different stock. The eBay sellers are only buying full lengths and cutting them up, making a big markup on price.
Paul.