Where is a common place to get sheet metal? Is there a certain thickness\type I need to look for?

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[Sold] 2004 X5 4.4i - Sterling Gray - Truffle Brown, Sport, Preminum, Rear Climate, DSP, Sirius, PDC, 132's (19 inch wheel), Comfort Seats
Dynavin, JL Audio Stealthbox, Alpine PDX-5 Amp