Ebay is where I bought my first pens before I upgraded to a blue lab and Eco tester2 ill be honest these are a very important instrument to have with hydroponics
I got both my first ppm and ph meter on ebay for around 20.00 total maybe 25.00 ph meter broke after one year but did ok and ppm meter still works but gave it to a friend just starting out cause I upgraded mine get liquid ph tester to compare to your ph pen cause the cheap ones are tough to calibrate. I still test mine with liquid too and check ph every day and ppm
ph keep around 5.5-6.2 I like mine at 5.8 ppm start off slow with nutes you can always add more.