If you're referring to a water testing kit to test your water's pH, ammonia, nitrite, and nitrate, I'd recommend the API Freshwater Master Test Kit
. It is usually around $17-25 depending on where you get it from, and its test results are much more accurate than the little paper strip test kits that are available. If you follow the link I provided, you can get it from Big Al's for $17.99 before shipping.
If you're looking for kits to test for GH
or phosphate, API also makes decent kits for those as well which are sold separately from their master test kit.