https://ghedirect.co.uk/product/vitalink-ph-test-kit-5-6-to-7-4/?gad_source=1&gclid=Cj0KCQjwpNuyBhCuARIsANJqL9NM6wRA5fSY_KhGVfvhlWKjCBgLnrXQF0mhtqhgd9B5mDfr0FzS2fMaAqhDEALw_wcB
That's a ph test drop kit.
But if you want, and I don't know why I haven't, once you've got the vial , you can buy...