You don't necessarily need a ph pen. PH strips will do fine for a soil grow. Just have to remember that the water should be clear when testing with color coded kits. If you want to test run-off, run it through a coffee filter or paper towel first. You can usually get the strips at Home Depot or probably Wally World also.

Google "Milwaukee ph-600" and these pens are said to be good. They're about $20, so I'd buy a couple of them at once, in case it does fail. (like 99% of pens eventually do)