They are not easily available and extremely expensive. Google for knurled thumb screws.
http://www.grainger.com/category/thumb-screws/screws/fasteners/ecatalog/N-8nd?perPage=32&requestedPage=2#nav=%2Fcategory%2Fthumb-screws%2Fscrews%2Ffasteners%2Fecatalog%2FN-8ndZ1z0nm46%3F_%3D1397241353612%26perPage%3D32