The washers at my local hardware store that say grade 8 SAE but they are not the correct thickness or hardness. Fastenal's are the same. Earlier I could not find and ARP listing for a washers-only set but I decided to check on Summit and they popped up there.
The Caterpillar dealers usually stock a lot of the fasteners that haerdware stores etc, don't too. I've found quite a few heavy and extra heavy washers, heavy nuts, etc, at mine.....or they can look 'em up by dimension and usually have 'em in a day.