If you have a "Tractor Supply Company" near you, they have a very nice selection. They also sell online, but no shipping if you can find a store near you.
I'm with Yvonne- the shipping will kill you- they are not the sort of thing you send by UPS or FedEx. Find your local farm and ranch store or feed store- Tractor Supply (TSC) is a great place for a lot of cool stuff and usually has good prices as well.
Tractor Supply, Agway and other agricultural supply stores are a good source for stock tanks. If you don't have any stores like that near you then you may want to check with greenhouses, landscape supply stores and even construction supply stores that handle concrete equipment. Those sources often use large tubs and vats in their trades for various tasks. I have a few 2' x 5' heavy duty concrete mixing tubs that I used to use for small outdoor ponds and bogs. The difference between them and a stock tank is that they are low sided so depending upon the application that someone may want to use them for they might end up having to build higher sides for them. Just another possible source for you.
Local feed stores, check with pet supply stores for ordering.. even walmarts.. they may not carry it, but can get it if paid for in advance. Home Depot also. They make metal and Plastic ones.. look for the Sheep ones.. they are not as tall.