The recipes recommend calasparra or bomba rice. Neither is easy to find. My pantry has long grain Basmati, short grain Kokuho, and some Arborio. Which one is the best substitute? 
The best substitute from the three rice you have on hand would be the short-grain Arborio rice. Both of the recommended rice are also short grain rice from the same general area geographically.   Basmati would be the worst choice as the it will result in the most difference in the texture of the rice and you would have to change the liquid content in your recipe to accommodate the Basmati rice. 