Shed materials

[00tec]

Looking to build a shed when the "stimulus" comes in. I want to put it on a 2x6 steel base like the commercially available options have. But dont know where to source the materials. I need something like 11 2x6x16 hot dipped steel joists. Anyone have a source for such things? The big box stores dont have much of an inventory, and I dont necessarily want to order a truck load of them. I just dont want to worry about the base rotting out.

Shed dimensions will be 12ft x 16ft