Pontiac Steel - they have a decent drop pile too, so you don't have to pay the cut fee or buy a full length..
Some say Metal Mart is pricey, but they charge a decent price on drops. I just got 16 ga sheet 24" square, and 24 x 30" for $10.. dunno if that's a great price, but a hell of a lot cheaper than buying littel bits at Lowes or Homies..