...maybe use 2 sheet of the aluminium sorta like 2 ply loo paper
Nooooooo!!! that old waxy stuff they put in public toilets is bad enough, aluminium would be agony!
On a more serious note ebay can be ok for all sorts of engineering materials in small quantities. Also possibly worth asking politely at your nearest engineering shop doing aluminium fabrication as they may well sell you offcuts.
If you are looking for larger quantities of plastic then these guys
are worth a go, but the postage is the killer unless you order enough to get free shipping.