Cole Parmer has a good selection: http://www.coleparmer.com/

(They are available internationally: http://www.coleparmer.com/dealers/default.asp )

For example a nice double junction probe:
http://www.coleparmer.com/catalog/pr...sp?sku=5900170

A couple of related articles you might find useful:

Measuring pH with a Meter

A Comparison of pH Calibration Buffers

Rich