It's called a "brix kit". I got mine from my service guys, and was not charged for them (two in the past 20 years). Make sure you get a nozzle adapter that matches what you have at your theatre.
If your company owns the equipment, and service calls are included, it costs THEM to come out and brix your heads. It's a reasonable angle for them to consider that you can keep an eye on it yourself if they leave a kit for you.
On the other hand, if you automatically have your company come out once (or twice) a year, make sure they check the brix on each call. A decent fountain system should go at least that long without any drift, barring actual problems with a valve.
Remember that with some valves, temperature can have an effect on the amount of syrup that flows at the nozzle. If you have the type of system that has the syrup/water lines running through the base of the ice bin, ice it down well before a service call. If yours is self-refrigerated (and always on), it shouldn't matter.