For shims, go to a machine shop or good auto parts store, they should have brass shim material in .002" increments or less.
I just ran across the same problem with a Nikon monarch 6.5 x 20 with target turrets. Same rings as my Docter 3x9 with no problems, but had to adjust it all the way up to get my bear loads sighted in. ??
I can use it the way it is for any shot I'll take on a bear, but it did make me wonder why it happened.