Where do I find an ....

I assume by “pressure test” you mean a test of the engine cooling system, as would be done with a radiator cap tester, not checking the air pressure in the tires, engine compression, or something else.

Toyota’s Repair Manual recommends two special service tools, the Radiator Cap Tester, 09230-40010, and Radiator Cap Tester Adapter Set C, 09231-40030, but these don’t seem to be readily available.

Stant Corp. says their adapters 12024 and 12025 fit the cap and cooling system, respectively, on the 2010 Prius, and if you enter one of these part numbers on their parts lookup website and click INTERCHANGE, it shows several alternatives. There a brief description on page 270 of their catalog (PDF).

I have no experience with any of these adapters, and I don’t know if the Stant adapters would work with your kit, but I hope this information is helpful.