I'm interested in finding a good solution to this too. I'd really like to be able to offer this in my shop, but with space & budget limitations, it needs to be compact and affordable. If anyone has suggestions, please share them!
After that all you need it to buy the tap equipment and a CO2 tank. Use cold filtered water and follow the instructions and you'll have enough carbonated water to make a lot of drinks. The nice thing about using this type of setup is that it is not that much more then buying one of those home units for half as much and will cost you twice as much to operate. Probably have a much higher failure rate as well.
This is about as easy as it gets. Otherwise you can go with a full setup with a motorized carbonation unit, cold plate, and the whole shebang. More expensive for the initial investment but a lot easier to make soda water on demand and can also be expanded out to run other syrup lines too.