no need to buy anything really. its water with a KH of 4dkh (you can make this by adding baking soda to distilled or RODI water), and bromothymol blue as an indicator (which is the indicator used for pH tests, APIs test kit is a few bucks at any store).
i happen to have 4kdh standard solution I can use that? How do I use it in my drop checker? According to my Chinese drop checker instructions I use 5ml Solution (in my case 4kdh) and 15 ml of tank water.