In my area anyway, the local concrete plant, one of our major aggregate suppliers, and my primary lumber yard all carry them. For a one time use for something like that though I think that I would be inclined to call around to some of the other HD's or contractor's rentals.
In my area I would go to Sunbelt. They rent a lot of construction equipment to professionals and DIYers too. I looked up the price online and it says $56 per day. This is for the motorized type that vibrates.
When I rent there I just pay with my credit card and I believe the card serves as a deposit in case you didn't return your rental on-time, damaged it, or you didn't refuel it. Their rates for fuel are pretty reasonable too; they do not gouge you too much.
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.
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