Any probe should work. Annual replacement of most any Ph probe ensures accuracy.
The manual should have instructions for makinng reagent from lab grade acid of a set molar concentration.
While I use a KH guardian, they are very similar in operation. I actually want a kh carer instead due to not making you buy proprietary Reagent, kamoer is a great company, and the app is likely nicer than a web interface.
Fwiw, I use a $20 Amazon ph probe, and it works exactly the same as the $90 oem probes. I replace them every 6 months becuase they start to drift just a tiny bit by then. Calibration should be at least every 3 months. Monthly is better.
I also use the Amazon cheap probes for the tank pH. Pinpoint probe would flicker ±0.01 ph units, so I switched back, and won't spent extra on something that is no better.