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Something basic, like this:
https://www.amazon.com/VIVOSUN-Meter-Digital-Tester-Water/dp/B01M5IASHD/ref=sr_1_2_sspa?__mk_es_US=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=ph+pen&qid=1596843406&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzUzZQTk9QSFJGVFJNJmVuY3J5cHRlZElkPUExMDIxNjYwM0ZEMFZHUk9CWUM0TiZlbmNyeXB0ZWRBZElkPUEwNTM3MTQyMVo4QlVWRjdCVUk2UCZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=
No need to get an expensive one, you aren't setting a lab.
I have a hanna one, was 40€ and its working fine, you can probably find it cheaper on amazon tho, just be carefull to not buy a EC meter instead by accident, but you should get a EC meter too, or a mixed PH and EC meter.