We use Coke, and I buy my syrup from a 3rd party foodservice supply. Amazingly, it's less expensive to get it that way than from Coke themselves... go figure.
Coke's installed and serviced the equipment at no cost to me. We're on our second set of dispensers & carbonators. The last set ran almost 15 years before it became too cranky to be worth fixing. They've also replaced all the lines with ones that are teflon-coated. Now, if I want, I can change flavors without having to replace the lines.
One thing I mentioned before, but is worth repeating: If you notice any off-taste in a neutral drink, like "Sprite", install a filter in your water line before you do anything else. Same with your ice machine. Chlorine will mess with the drink quality and carbonation levels. Stick your head in your ice machine & take a good whiff (Yah, I know... funny). If it smells like a swimming pool in there, a filter will make a major improvement in your drink quality. We fought that problem for almost 20 years before I put filters in. Coke, who should have known, was no help in this area. They replaced a lot of stuff that wasn't necessary.
I got my filters & replacement cores from Home Depot... something like $30 each. The replacement filter cores are only $5 or so, but don't need replacing very often.