FYI rigrite is out of the isomate internal shuttle blocks... I need a replacement and a spare but cant find one. If anyone finds a suitable replacement (especially with ballbearings) please let us know. I live in Wisconsin and have visited Harken HQ and they do not have something that will work..... Back to the drawing board.
I restored single line reefing to my boat, which required finding two shuttle blocks.
I found two 42mm (sheave diameter) shuttle blocks made by Sea Sure, available for sale from Sea Screw, Ltd., in the UK I got the last two they had. They currently have two left of the 28mm size.
The are nicely made, with "densely packed carbon fibre needle roller" bearings. Thy have rubber-like material housings so they won't make a lot of noise inside booms.
Here's a Sea Sure catalog. Search for 09-60 and 29-60.
http://www.teufelberger.se/wp-content/uploads/pdf/SeaSure.pdf
I found Sea Screw to be very reliable, with very quick shipping, even though they are in the UK. They gave me a big discount for buying the last two, and I didn't ask for it.
Here's Sea Screw's website page with the remaining blocks:
https://www.seascrew.com/browse.cfm?s=Block Inside Boom Single line Reefing
The Sea Sure blocks may be available elsewhere, I don't know.
I'm very happy with how these worked out. I can very easily reef from the cockpit with a single line. I have the option of putting the reefing line on a winch, but haven't found it necessary yet.
Good luck.
Update: these guys seem to have both sizes:
http://www.seapost.co.uk/seasure-09-60-inside-boom-single-line-reefing-block-12705-p.asp