I've been told that plastic drywall anchors make dandy snap caps, but I've not tried it, and I don't remember which size I was told to use.
Another possibility is to use Aguila Colibris. These are super-quiet primer-only rounds that use a full-length .22LR case- unlike .22 Short CB caps- and are loaded with conical 20gr bullets. They generate virtually zero recoil and are great for practice.
Aguila also loads a similar round called the Super Colibri, which is loaded slightly hotter to guard against squibs when fired in a longer rifle barrel; these aren't necessary in a PT-22, but could be used if you can't find the standard ones.
The main downside of both rounds, however, is that they categorically don't generate enough force to cycle the action of a semi-auto, so the slide must be cycled by hand. This may introduce a related problem- the conical bullets may not like feeding from the magazine, which could force you to single-load them, making practice highly tedious.
Bottom line: Don't invest in multiple boxes until you check that they work! Also- although it may be tempting since they're so quiet- they use lead bullets and therefore
shouldn't be fired indoors without ample ventilation.