Using it for making actual root beer, or for cooking? My husband has done a lot of research on RB...
...and we make it and dispense it from our cornelius kegs at camping events every summer.
If you want it for making root beer itself, he suggests a brand called Gnome. It's been very good! A mail order place in Minnesota has been where he's been getting it from. Lemme ask... he says he got it at Northern Brewer (
http://www.northernbrewer.com/soda.html). But I see that other places have it, too.
Northern Brewer says: "Anyone who's tasted Gnome Root Beer doesn't need to be told that it is the creamiest, richest-tasting root beer that you can make at home. If you haven't tried it yet, make a batch and see for yourself—we think you'll agree. The extracts come in 4 oz packets and yield between 5 and 10 gallons of soda, depending on how strong you want the soda to taste."
Williams Brewing has it, and says: "Gnome Soda Extracts are the finest available, for a rich, old-fashioned flavor. Gnome Soda Extracts are made in small batches in Oregon, and are available in traditional flavors hard to find in commercial sodas. Packed in glass to preserve freshness. One 4 ounce pouch (and 2.5 pounds of corn sugar or equivalent) will make 2.5 gallons of rich soda using the traditional mix and carbonate method. Or, you can mix them fresh to taste with our Seltzer Bottle (see link below) adding a teaspoon of extract and some sugar to a glass before adding the carbonated seltzer and ice."
http://www.williamsbrewing.com/GNOME_ROOT_BEER_EXTRACT_P985.cfm
Doing a search on "Gnome Root Beer" on Google yielded many results for places to get it.
http://www.northernbrewer.com/soda.html