I bought mine at Amazon.com. It is sold for making beer, costs about $40. People who wrote the reviews often used it for testing their pool.
The problem with testing dry foods is that many don't dissolve in water. Also, water itself is usually neutral (pH of 7), and that will bring the pH of the food toward neutral. So it is difficult to find pH values for many dry foods. Nuts and rice are not included on Koufman's pH list in her book. I eat a lot of brown rice, and it seems to neutralize many foods I eat that may be on the acidic side. I don't get heartburn, but my stomach is very sensitive, and I can tell when the meal I've eaten needs to be neutralized. Most evenings around 6:00 I have a glass of skim milk and a couple of graham crackers. That seems to settle my stomach nicely. But I still am very careful especially to keep fats quite low.