I just checked McMaster & found nothing useful -- the only extra-thick washers had 0.281" holes.
A 5/32" to 3/16" thick washer made from 6061-T6 works great -- basically, make it a bit thicker than the stock steel washer so it won't deform under pressure, with a hole that just barely clears the threads. You can make buttloads if you're not afraid of using a cutoff tool. But, it's absolutely not worth the lathe time unless you enjoy that sort of thing.
Oh -- any old grade 8 1/4-28 nut should work, but I would hesitate to use a grade 5 nut. A grade 8 SAE washer may work OK -- anyone want to try?