Depends on where you live but any gardening center should have them. People who deal with irrigation and water should have them. But you need to go to a place that specializes in gardening or water or something around those lines. I got mine at a gardening center that sells the works and it was a GREAT investment. There is no more blindly guessing. You put the needle in the water and it gives you a number and you adjust accordingly. Would highly recommend one!!! buying from a garden center would prob be around 100$ for a TDS meter though. Probably cheaper online
The Filter, pitcher and pen that comes with it totals like $30. The tester it comes with isn't going to be of very good quality. AFAIK it can't even be calibrated.
I have an Oakton pH pen and an Oakton TDS pen. I think each one was $79.95, they sell combo meters as well though. Just stick with the good brands. Hanna, Oakton, Milwuakee, etc. Don't skimp on your test equipment, you need acurate results.