Bellforestproducts.com has them, $1.50/ea. Shipping for just a few pen blanks may seem a bit high though. I just received some beautiful E. Indian rosewood along with some other beautiful turning blanks from them. Shipping time is good and if you want to buy other spindles, shipping is free on orders over $100.
Rockler sells them for $2.49. I see them at my local Rockler store, but never turned one. My friends tell me that it is tricky to turn, with a tendency to splinter and fly off the tubes: https://www.rockler.com/leopardwood-pen-blank
I will be turning a cat twist pen later today. I wanted a "cat" wood and thought about using leopardwood, but chose tigerwood (Gonçalo alves) instead.