Bromo blue is what you want to use. You may order it from the 4 KH ref solution vendors or, get it off ebay if you want bulk.
Realize that there is a trade off with the drop checker method: it's only as good as the color resolution between the pH color scale for bromo blue (and many folks only look for 3 colors, blue, green and yeller........ the variation is huge there) and it's takes about 2-4 hours of lag time versus what is actually in the tank for the drop checker to respond.
While convenient, the method is hardly precise, but has no interference from other forms of alklalinity(a +) but the accuracy and the response time are very poor( a -).
You might use a mixture of measures until, a reasonable priced CO2 meter becomes available. The cheapest is still 2500$
And you can hook it up to a universal controller and regulate CO2 taht way and not need to calibrate the probe for 2 years.
But that would cost 3000$ and take some technical know how.
I'm adding one such set up, but heck, unless it's 10X cheaper, no one is really going to bother.
However, the system is available and I have installed it.
pH/Kh charts and KH test/pH meters can get pretty close, but assumes that all the Kh is due to carbonate alkalinity. In some tap waters, make up water, this is true. But in others, it's way off.
The other issue is folks rarely measure throughout an entire day cycle
They take one reading per day to check.
Ideally, you would take readings every 1 hour or so.
Then you can see what the CO2 is doing throughout the entire day, otherwise you can miss poor CO2 ppm's in the morning but have high levels late in the day when you come home etc.
CO2 is ephemeral, and changes far faster and is more important to plant metabolism, biomass/growth rates and uptake than any other next to light(which in our tanks, it very stable overall).
So most of the focus should/would wise to be on CO2.
Plenty of folks have drop checkers and poor algae ridden tanks.
You need to keep better tabs than assuming that will cure most of the issues, it's great for a general quick rough glance.
Do not put too much faith in it however.