Sheet metal isn't all easy to find, but with this idea maybe it'll help.

For Smelting:

• Empty soda can/Empty canned food = "small metal scrap" for melting

• Cooking Pot/Frying Pan = "medium metal scrap" for melting

• 1 small scrap per can and 1 medium scrap per pan/pot

• 10 Small scrap to make 1 metal ingot

• 5 Medium scrap to make 2 metal ingots

For Forging:

• 6 Metal Ingots to craft 3 Sheet Metal