Used - junk yard or anyone selling a KA for parts
https://www.courtesyparts.com/?mid=0&pa ... rch_in=all
FR sport has a couple of sizes that don't match what you're after, but damn are they expensive. Courtesy lists the part # but you have to decode them and they aren't listed as in stock... maybe call and see if they have what you need? Get the larger ones and plane them to size? That's still going to be a pretty penny. I'd vote going to a junkyard and looking for Altimas with the KA, Xterra from 00-04 had them, and Frontiers from 89'-2016. I tried searching for Frontier valve shims but hit a wall there, might be your best bet though.
The machinist who did my head didn't have to change shims on a 3-angle regrind and BC cams upgrade so I guess I lucked out. Best of luck man. It'll feel great once you get it back together and start it up for the first time.