My favorite ones are at Green Leaf Aquarium. Very good quality and several shapes to choose from. OS,
I've researched drop checkers and are all the same. Inaccurate and gives all of us a false sense of security. But I have one. Lol. eBay
The drop checkers are not exactly accurate but it's a heck of a lot better than nothing. After a time you get to know what exact color it is that works with your fish, plants and size tank. The GLA ones are pricey, but are of thicker, stronger glass and note that a good drop checker will last you a life time. OS.