I have heard from reliable sources that the replacement for sassafras in some industry extracts is actually camphor, but most of the camphor that I have seen available says not to take internally. I'd guess it's probably added in ppm or ppb amounts.
Also, if you read the ingredients on the McCormick extract available at your local grocery store I believe it says cherry bark extract.
Wintergreen is used in some root beers, but it's not the only component. Like a cola, it's a very complex flavor. If you're trying to recreate it from natural ingredients, then there's more than just one thing that you'll be making it from. Some of the recipes that I've seen include licorice root, dandelion root, sassafras root, anise, raisins, birch twigs, citrus peel, molasses and wintergreen oil.
All three of the roots listed above I've purchased at
Good Earth Natural Foods when I lived in UT. They have some liquid extracts available online. In store they have actual roots and herbs including things like hops and elderflower at some of their locations. If you can find a store similar to this in your area, I'm sure that would be the best place to start.