Mylar shim material

[Martin]

during my windsurfing days I used to cut mylar shims for the fin box to make for a more solid fit.
I don't remember where I bought it and could use some now to make a non windsurfing/kiting related shim.
Any ideas on where to find it?
When I googled it, all I seem to get was mylar for "grow-ops", which of course I know nothing about!
martin