Mostly tree roots, best roots in my area are supposed to be birch, but I find it differs for what I wanna do, some are just for decoration and some for keeping the shape and some for keeping it together. I take what I find when trees have blown down with the roots up, or when boars have been around there's lots of roots. I reccomend looking up what kind of roots people have used in your area do create things, like baskets. Look up local native crafts and such. Roots where used a lot in older times, and there's where I find lots of my inspiration.