You can usually find a simple one at a pool store or an aquarium supply shop.

If you want to get more serious about testing your water quality, these kits are the way to go...they have all the major water quality tests and a good instruction book: http://www.hach.com/hc/search.product.de...azJOREF3TQ==%7C ...but if you only use it once or twice a year, it probably isn't worth it.