Since people have been interested in getting their own, there's not a whole lot you need. There are several soda siphons available (I have the iSi Soda Siphon Brushed Aluminum 2248), and then as far as cartridges go, you can often find them at restaurant suppliers or wholesalers (e.g. Cash and Carry). My most recent shipment came from Creamright, which specializes in these things (they also have N2O cartridges for making whipped cream and/or getting high); I just use their generic CO2 cartridges. They also have slightly better deals on siphons, too.
Anyway, the other part of the equation is that after you have a source of seltzer (and this works with bottled seltzer too), you can make your own soda with Torani syrup, which comes in a bunch of flavors, and is sweetened with sugar instead of HFCS (and they also have diet versions). I just buy that stuff at BevMo.
If you want soda that's brewed in a more traditional way, there are plenty of recipes out there for home-brewed ginger ale and root beer using a fermentation process, or you can cheat and get (or make) an appropriate syrup and mix it with seltzer (typically by making simple syrup and combining it with a flavor extract). That does generate a slightly different flavor, though, and a fermentation approach also produces a bit of alcohol (not enough to get drunk, but enough to cause problems if you have an alcohol-sensitive medical condition).