whats to stop you making your own with some ph up/down and a freshly calibrated trunchion?

stability of the DIY solution.

Im also under the impression you should never mix the two.....but im sure someone will confirm or reject that.

why would it be unstable , surely if it were in a sealed bottle it should keep ok.
i suppose there is only one way to find out..

some bottled water display the ph on label, have a look

maybe you could use that then to calibrate.

just an idea

PH calibration fluid isn't expensive.........thats what stops me
and obviously it would be more stable