G'day Iain.
Make your own.
I use 90mm poly pipe and hose clamps. Cut a 1,1/2" piece off the pipe then cut it, wrap it around piston & mark to get the right length (1/4" gap is best). Once the rings are set in position and oiled wrap the pipe around and compress with the hose clamp till tight then back off till the pipe can just be rotated. Push the barrels down till well over the rings, undo the hose clamp completely and remove the clamp & pipe over the barrel skirt so not to damage the piston. I also cut a nylon cutting board to fit under the piston skirt to hold them still. Job done.
Cheers