3M's Safetywalk comes in rubberized and mineral abrasive adhesive tapes. Like Greg said a skate shop may be a good source. Their adhesives typically work!
http://solutions.3m.com/wps/portal/3...=6702767&rt=c3
Home centers may carry the rubberized meterial for non-slip tub/shower applications. The mineral products are sometimes sold for exterior deck step and ladder applications.