I first discovered teflon 'bone' folders, funnily enough, when I took a traditional bookbinding class at the Center for Book Arts in New York. My teacher was a disciple of the Alabama Book Arts program and a traditionalist through and through, so I was surprised when she recommended the teflon folder to us. She had a collection of bone, horn, and hand carved folders but the teflon folder had become her go to instrument because it doesn't leave shiny marks or any traces of its presence on book cloth, leather or decorative paper. With a teflon folder, there's no need for the pesky layer of newsprint between your tool and your work in progress.
I bought my first teflon folder at Talas in New York. Talas is a varied and unique source for Book Art supplies, specializing in hand bookbinding tools and resources for conservation. Even if you're not conserving 17th century manuscripts, they're still the best source in New York for tools of the trade. They sell two sizes of teflon folders, each with dual edges starting at $19.50.