If you want to try your hand at making your own root beer, here some companies that supply kits. Results have been mixed.
LisaN bought a home root beer kit from Leeners and says it contains everything you need. The mixture needs to stay cool, though; a heat wave compromised her efforts!
Ricepad used the kit from Hearth Song, and immediately produced “the best root beer he ever tasted”. Subsequent batches were a little less successful.
ChrisVR has used beer-wine.com for beer brewing and cheese making supplies They have a soft drink kit, as well. The staff there is helpful and informative.