You can buy in-line TDS meters for the RODI system, and a pressure guage. Loads of places sell these, but I've always used Vyair, as their customer support is excellent.
If you mean test kits for ammonia, nitrite, nitrate, phosphate, cal, kh, mg, etc. then RedSea is a good brand, or Salifert, or Colombo. For Phosphate I like RedSea, but there's lots of comparisons and reviews on here if you have a search, or look in the Chemistry board.