I use Zatarains Root Beer Extract with Lavlin #EC-1118 Champagne Wine Yeast. I've experimented with 4 different type of yeast (including Active Dry Bread yeast) and the Lavlin #EV-1118 brews out with less of a 'yeasty' flavor.
You can buy the extract here:Zatarain's Root Beer Extract, 4-Ounce Plastic Bottles (Pack of 12)
And the yeast here:
And here are some instructions on making your own rootbeer: