Doofus: Thank you! And nice username.
I did read the "what's your name mean" thread.
I got the metal at Lowe's. It's galvanized sheet steel. It comes in rolls, iirc it can come in 10" wide, 14" wide and 20" wide, possibly more sizes. I got the 20" wide as it fit my needs (and you can always cut it smaller, but hard to cut it bigger: you can rivet segments together if you want but it's easier and faster to just start with the right size if you can, obviously).
The hardest part about working with it is cutting it without cutting your hands, and cutting it in the right shapes (a bit difficult to cut nuanced curves in sheet metal with tin snips), and then dealing with the sharp edges (I've started using flat-grip pliers to bend over about an eighth of an inch of metal to 90 degrees, a flathead screwdriver to fold it in to an acute angle, then a hammer to flatten it out: you can see what the finished result looks like in my pic of the airdam/undertray while it's off the car in an earlier post).
However, it's -great- for being able to bend into shapes you want it in, without needing a heat gun or anything like that, and if you get it wrong you can just bend it back (with the help of a hammer if necessary). It's thin and doesn't have the corrugated center, so it's easy to bend, and to bend small bits. It's also quite sturdy if even somewhat supported. It's easy to connect to itself, or other objects, with pop-rivets. You don't have to worry about heat at all (the undertray I put on my car is a couple of inches from the exhaust headers), nor do you have to worry about it possibly getting more brittle over time (due to sun exposure or whatnot, like some plastics might).
I've had 5 sheet metal patches on my car, the older ones for over a year or two, and so far only one of them has shown any rust at all. So rust is definitely an issue, but i imagine this could be mitigated by slathering the cut edges with rust bullet or something similar.