it ticks me off , when one of the reagents runs out , and the reat of them are two thirds full . You have to buy an entire new kit . You can't replace one of the reagents
Sorry, I can relate with your frustration. However, if the titration reagent runs out, it probably means you were provided an excess of the other reagents.
The titration reagent is easier to make than just DIY'ing your own test kit. You'll need some chemistry background and basic lab equipment (if you don't have access to a lab) where you'll need to do some calculations, and you'll need to be able to confirm the reagent's accuracy. These are things most hobbyists aren't able to do. Perhaps one day, a supplier will be willing to provide reagent refills, but I don't think it'd be worth my time.
As an aside, my 9yr old (with LOTS of help) made rudimentary test kits (Alk, Ca, Mg) for a science project, but he enjoys learning chemistry.