about isolator for PH probe

By "isolator", I assume you mean a high input impedance amplifier? I know pH probe have a very high impedance, so nearly any current draw will create an error. I don't know how high the impedance needs to be. This may require a circuit with a bootstrap and guard rings. Do you have a schematic? Perhaps you can make your own. Have you looked for sample circuits?

Their OEM pH board doesn't look unreasonably priced to me:

A working approximation is around 1G worst case.

You would probably get away without a guard band provided that the humidity and ambient temperature isn't too high. Many opamps the input bias current rises quickly with temperature above 25C ambient.

If you don't care too much about thermal drift or precision calibration then almost any modern FET input opamp will be adequate in a unity gain voltage follower configuration. eg #9.3 in TL071 datasheet

Or a more recent automotive part like

If you do care about precision calibration and have to ask this question then you are probably much better off with a commercial product.