You can buy angle steel (maybe Al too?) at HD but it is over-priced.
I'm told that a welding supplier would be a good local source, but I have bought Al online from this company www.midweststeelsupply.com
and it is cheap. I got 6.5ft lengths of 1.5" x 1.5" x 1/8" angle aluminum for $15 each and shipping on ~dozen of them was I think under $30. Only took a few days to arrive.
Just having a piece of metal is not as good as having the power tool. But if all you want is a big, thick straight-edge, you can certainly buy that.