what is a good substitute for bomba rice?
Any medium grain rice would work. Goya store supermarket brands sells a "Valencia" short grain rice meant for paella and a medium grain rice. Arborino can be substituted but dont over stir so the glutinous starch doesnt take on a creamy-like consistency. Basmati is too fragile to hold together, sushi rice - too sticky, and long-grain rice wont absorb of all seasonings and flavor.
The short answer is short grain rice. Stop. Arborio or carnaroli are okay depending on how you intend to use it. If this is for paella it shouldn't be stirred anyway. Long grain rice=total failure. If you need to you can get by with a Cal Rose type.
I make Paella all the time and if you don't to buy bomba rice which is the easiest to cook with, also calasparra rice or valencia rice will do. I get mine from rapososgourmet.com or through their site at Amazon.com, they have the best prices and customer service.