MATT - Doesn't surprise me McMaster-Carr has those shim washers. As a very astute Purchasing person told me 20+ yrs ago, "McMaster-Carr is an excellent reference on what's available, but NOBODY buys production parts from them!".
Even when your BEST local hardware store doesn't have the bits & pieces to fix or repair what your working on I have ordered quite a bit of stuff from McMaster.
Back in Son & My R/C racing days we switched ALL our R/C cars/trucks over to TORX machine screws, both socket head & pan head, thankfully they were all #4-40 and less than 1" long, boxes of 100 pcs of 1" long Torx cost less than a bag of five screws from the local hobby shop.
Speaking of shims & shimming assemblies, I buy pkg's of assorted brass shim stock, .003" to about .015" thk. sheets are about 3-4" x 5-6". As long as there's no fast movement against the brass they hold up great. The thin stuff cuts with a scissors, I use tin snips on the thick stuff.
And making shims thinner, BT-DT. You know what Differential rings are in ball diff's in R/C cars? stamped rings about 1" OD X 3/4" ID and maybe .050"-.060" thk, hardened, ground, & polished. I used to resurface them with WD-40 on 1000-1200 grit Wet/Dry sandpaper & repolish them. A new pair at the hobby shop was $6-$8 for two rings. We kept 2-3 spare sets and I'd resurface them all at one time, we ran a new fresh set every week on the dirt off-road track. I always referred to our race team as "TEAM CHEAP" for good reason!