Shims

Depends on the tolerances needed.
I've got aluminium foil at thicknesses of 0.03mm and 0.01mm for fine details. :)

I've got access to (in increasing thickness) kitchen foil, some copper foil my wife uses for craft stuff, aly drink tins and an empty 3l olive oil tin (kept it just in case). For accuracy I've got a micrometer. Will that do, do you think?