i got mine off of ebay. I have a pH pen and a TDS pen, not a combo deal. The pH Pen was $30 and the TDS was like $15. here's what i have and they work great:

pH: NEW DIGITAL PH METER FISH MARINE AQUARIUM, HYDROPONICS - eBay (item 380039909803 end time Jun-29-08 18:38:08 PDT)

TDS:
HANNA Primo TDS Meter, PPM Tester, Conductivity - eBay (item 270248851582 end time Jun-29-08 13:25:30 PDT)

good luck
Skeet