John, I always direct people towards local sources for plate, as the shipping can get expensive. If you don't mind paying for shipping and want to save yourself the trouble of tracking down plate locally, then this place here seems to be pretty competitively priced:http://www.onlinemetalstore.com/items/A36_Hot_Rolled_Steel_Plate.cfm
You're looking for hot rolled steel, also called 'mild steel.'
Since quite a few home ovens have a peak temp of 550, I strongly urge people to get 1/2" steel, since 550 + 1/2" steel will give you any bake time from 2.5 minutes up. Once you start trimming away the thermal mass, you're tossing the dice on bake times. 3/8" might give you a respectable 4 minute bake at 550, or it might not. 1/4" at 550 has failed to rise to the challenge, but the jury's still out on 3/8". Scholarly speaking, it would be nice if someone did bite the bullet and test 3/8" but I don't think anyone wants to be the guy/girl that finds out it doesn't quite make the grade.
As much as I extol the virtues of steel every chance I get, I think it's important to be aware that it's not a one size fits all solution. It is, based upon the number of people that like NY style pizza and who own electric ovens that will hit 550, a one size fits many, but not all. Theoretically, a thick enough piece should be able to do Neapolitan bakes (with close proximity to an electric broiler), but right now the only tried and true application where steel plate is thoroughly tested is for NY style in electric ovens that can reliably hit 525-550.
I should also mention that I'm working under the assumption that the plate purchaser wants a plate that can do relatively quick bakes without mods/tricks. If you're open to tinkering and pushing your oven to, say, 625, then 1/4" plate should suffice for NY style. I've been brainstorming easier oven tricks lately, and I think I've come up with a way to get a half decent bake out of 1/4" plate, but, so far it's very preliminary and theoretical.