This turned out kind of amazing.
I called up the local BMW bike dealer to get a baseline on the shim price.
I got this guy, he says "you sure it's the old K100?", I say it's an 86. He says it's the first BMW he worked on, the bike he started on as a mechanic. And then he says he has a shim kit, his personal one, not belonging to the shop. And I can just borrow it. Free.
I show up, expecting to pay a deposit, or leave something as collateral. Nothing. He just hands me the two boxes and says to bring them back tomorrow.
So, I got my valves set for free with OEM shims. And the bike runs nice.