Phone Hanna Instruments in Johannesburg (Bruma lake area...)
I suggest that you get a pH pen with a replaceable electrode - it might be more expensive to buy initially, but will work out cheaper in the long run, as the electrodes only last a year or so (perhaps 18 months at a push...)
Also make sure that it is possible to calibrate the pen - many types cannot be calibrated, and that makes them more than useless.
Hennie