I bought a used set of R8000 rim brakes. They didn't come with the anti-rotation spacers. I don't know if the spacers are a critical part, but they are sold out everywhere in the US. (Y85850120).
they are 2mm thick. I have found two 3mm thick washers ( don't know the part number, I think I can shave down to 2mm. Is this a critical part? Do they have to be 2mm thick? 1mm doesn't sound like much but I wonder if it will push the calipers too far back (on the rear) or too far forward (on the front). I can get some from the UK but $35 a piece? not just no but h*** no! I guess I'll just try the 3mm and see what happens...Wonder if I can just shave 1mm off the 3mm ones? Anyone else with this problem? TIA

Try your local bike shop. They may have various thicknesses on hand and for a reasonable cost.

Don't worry about such a small difference. Many times an addnl spacer was put in to get brake pad clearance to the fork/seatstays.

Just need to check that you still have enough engagement of the nut through the fork/bridge. Shimano recommends 5 turns of engagement, (probably very conservative).

Most thread strength guide lined claim that after 4 threads of engagement the "strength" increase with each additional thread of further engagement results in less and lass increase, of "strength". Or in other words, 5 threads should be enough given good thread forms/profiles.

The anti rotate washers are not really needed but I still do like to use them. More than a few times I've used simple flat washers in place of the serrated anti rotate ones. I do make sure the caliper's mounting bolt and nut are properly tightened. If the bike has a "classic" external cup headset take care of caliper/cup interferences to insure the caliper sits flat, which helps insure proper tightening ability too, besides the avoidance of friction when steering. Andy