I hear the bark can impart a more citrus flavor. Maybe that's what you're looking for? I'm not 100% sure though.
I know ebay has some pretty good sources for Sassafras Root. Just keep in mind the carcinogenic properties it "may" hold. As Sassafras isn't a traditional ingredient in root beer anymore.
check this out
In the 1960s, laboratory tests determined that safrole, the major component of the oil of the sassafras root bark, was toxic to the liver and could cause cancer in rats and mice. In 1976 the U.S. Food and Drug Association banned the usage of sassafras containing safrole in products for internal consumption.
Commercial root beer brewers and "extract" makers scrambled to reformulate their recipes, either balancing out the missing sassafras with other roots or synthetic flavors or by extracting the safrole from the sassafras root bark oil.
Although you will find sassafras oil, tincture, and root bark available for sale, they are legally intended for external use only.
I'm no doctor though
Hope you can figure out something that works for ya'. I'm sure in small amounts it probably wouldn't be TOO harmful anyway.