I just used 2 of the metal ones I got from the guy on FABO that I mentioned on the previous page. I'm thinking the next time I needed something from McMaster Carr to look and see what they have, I have 2 more here but they'll last me a couple of years at least before I put them to use. I looked at lower, Menards by me thinking either of them should have something in their hardware aisle but didn't see anything. I was surprised.
I'm sure a 1/2" id spacer would be good for lotsa other purposes than just as a distributor shaft stop.