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I just went to Partsource, and picked one up, it's the Lisle brand, 2 1/2" to 5" piston ring compressor. I found it was a little finnicky, but worked pretty well.
My favorite are the tapered style. The diameter is adjustable over a small range (about .250”) and they are pretty fool proof. The ‘pliers’ can be a pain in the *** and result in broken rings.