You should be able to order a box of shims from Goodson. I usually buy a box of .060 and another of .015. I don't have the part numbers handy (anybody have them?). You can just measure a guide a determine what you need. Also, the shims go on the "guide side" of the assembly.
https://goodson.com/collections/valv...-booster-shims
Another option is to talk to VanPelt or Michael at ThirdGen - they probably have them as well.