You aren't going ti find any washers at Ace that will work. As I said earlier, you may not need any of the shims. My 3996 didn't have one. But the replacement nut needed one, although I decided to put the original nut back in.
Prices at Sears were typically somewhat higher than prices at Clausing on parts that Clausing still has on hand. However, Clausing had the shims last year so I can't account for Sears asking $30. Unless they've got the part numbers screwed up and quoted you something else. I can't recall what I paid for the two I bought from Clausing last year but it wasn't anywhere near $30. If you need to buy anything else, like the nut, call Clausing. If you don't need anything else and you already have a good pair of ten snips and a set of Arch punches, the sheet of shim material you'll have to buy will probably cost about as much as the shipping (UPS has no increments under one pound).
In any case, the dimensions of 9-87 are as follows:
Length 1"
Width 3/4"
Thickness 0.011" (probably a nominal 0.010")
Hole is in the center.
Hole diameter 0.375"
Corner radius about 1/16".
