are you looking to paint on, or add to gel?
if painting, most of the non-skid is pumice (ground up lava rock). sand also works
I think I paid $5 for a 3# bucket of non-skid from my fiberglass supply house.
i have used it for the last 10 years on projects from the non skid painted porch to the bottom of the canoe, etc.
if adding to gel, mix up a heavy mix of fumed silica in the gel, roll it on with a heavy roller. let sit for about 15 minutes until it starts to kick, then roll over with a bed liner roller and pull peaks.