How to make the perfect root beer
I have long kept a copy of an old usenet posting on my web server that addressed the flavorings issue. There are alternatives to sassafras, and sarsaparilla plus dried burdock may get you most of the way there, and I always like the drink called sarsaparilla a little more than most mass market root beers. Birch beer has its charms, too. I think you can experiment with a few different barks and roots. Wintergreen and some of the other ingredients are used for other reasons; I'm pretty sure they were present in earlier forms of root beer as well.

See http://www.jagaimo.com/bistro/rootbeerfaq.html for the old discussion I kept a copy of (circa 1995). The same posting covered some suppliers but they may or may not be relevant 16 years after I collected that posting.