I didn't research the size, but just thought I mention that I saw these.
www.ebay.com/itm/Alemite-Standard-Button...&hash=item4ae505d591

he calls them standard which i believe is 5/8" head size. giant are 7/8" i believe and the size used on cats. both come in different pipe male thread sizes so u need to know what u need b4 u buy. i got a bunch of 7/8" by 1/4" pipe i think. 1$ ea. minimum of 10, + $5 S/H.

I think there 3/8" pipe thread.

I was out in the shop tonight, there is 3 new ones on the shelf. Measure approx. 7/8 across the head with 3/8 NPT on the back end!

New ones are available from McMaster Carr for $6.40 each. 7/8" diameter head X 3/8" NPT thread.

I did some checking and the button head fittings for the idler are not the same as the ones on the upper and lower rollers. Those are 3/8 NPT thread, but the ones for the idlers are actually 5/8-18 NF thread. The threads are the same pitch per inch, while the taper pipe fittings will start in the idler yoke, they actually are slightly (.030) bigger and will not screw in all the way. There are also other differences between them.

The parts book lists them with different part numbers so be careful what you get.