Try an industrial metal distributor like IMS Georgia Steel (they have location in Marietta). Since they will want to sell you a full sheet (4or 5 by 10 or 12 feet) ask if they have a "drop". A drop is a piece left over after they have sheared full sheets into some smaller shape for a customer. If they do't ask them who some of their large customers are (might even get the name to talk to) so you can approach them. Depending on what you need they may have some scrap large enough for what you need.
Also, if they ask you probably don't need "certs". They will probably charge more to give you a copy of the material certs which track the manufacture of the lot from the mill. Needed for traceability in certified product production - for us - not so much.
PS - these industrial sources will sell you want you need on a "will call" basis if you pay with a cc...
How close are you to Newnan GA? Robert and Sons Aluminum....That's where I buy pre-painted aluminum sheets in .040" to sell to my circle track racing customers. Off hand I'm embarrased to say I forget what alloy it is that I buy from them, but they may have various alloys, including non-painted, and I know they have a variety of thickness. How much do you need?