Take a metal file to the gears inside the diff, if you cannot file any shaving off of the gears there is very little chance you will grind those gears down. The diff gears are virtually indestructable. I reassembled my diffs withtout the shims as per CEN and I have no problems!!

However if you want to put shims back.....Oh by the way if you had shims in there before you torn it apart why not re-use the shims that came out?????

Anyway if you must put the shims back , you could find the shims on-line(eBay).