Hi jafnob, I buy mine from the reject shop and 2 dollar shops but keep in mind you get what you pay for. I only use them for shelf display where I know they won't get knocked. Cheap ones will break easily but work.
If u dont really care if they are a bit bigger , Hyclor has a testing stand at Bunnings and u can grab one to test your pool water with . Dont be greedy tho