The easiest spacer is one you make yourself. If you want a 3/8" tenon,
thickness a piece of wood to that dimension and drill an
appropriately-sized hole for the saw's arbor. Mark the spacer for
future use. I typically make mine slightly oversize (by about 1/32" or
so) to allow final fitting with a shoulder plane.

It's important when you are using a setup like this to use a
zero-clearance insert plate for your saw. You'll want as much support
for the workpiece as possible, and you don't want the cheek cutoffs
flying around the shop.


Jeff