Wherever you find it, get more than you need. It seems to come and go. It's really cool though, you can finish it rough for super-grippiness or sand it smooth for a cleaner look, and it's still BRIGHT orange. Tough as... well... it's really, really durable.
Wear a respirator and open a window when you cut/grind it though! It stinks to high heaven if you go too fast and the little bits of glass can't possibly be good for your lungs! If you cut or grind it WAY too fast it will kind of de-lam and char and shred apart and make a gawd-awful mess... don't ask me how I know, just be gentle.
Despite all my warnings, yes, it makes awesome handles.