Lyc IO-360 in my M-5. Gascolator occasionally leaks. Fixed by cycling it with the tester. Thinking an O-ring in there getting old. Are these just common hardware items any mechanic will have?

Most any auto parts store should have one as well as a well prepared mechanic

For less than $20 other than the shipping you can get a brand new whole drain valve along with a few replacement “O”rings from Aircraft Spruce.
If the threads look to be about 3/8” in diameter it’s 1/8” pipe threads if it’s threads look about 9/16” it’s 1/4” pipe. Once you have a spare it’s easy to swap them out , clean out the crud and or replace the “O”ring and always have a spare handy.

Not part of what you asked but if you plan to disassemble the gascolator be sure to get NEW GASKETS from Maule or 3/32” full contact immersions fuel stable rubber gasket material or it will likely leak . FYI most gasket material is NOT immersions constant liquid fuel contact stable

You might have dirt in the valve seat that is causing the leak. That's what happened to me several years ago. I saw fuel stains on the bottom cowling and my A&P and I removed the gascolator to inspect and clean it. The screen was partially blocked with fine dirt and there was some in the sump valve seat that was preventing it from seating fully.

If you replace the o rings, there are 2 on the original gerds model. One on the pipe, and one inside the hole on the bowl.

I posted on here a while back on where to get cheap gaskets for the top to bowl mating.