The type will usually be listed right on the package. The rice that I buy (Nishiki) specifically says it's good for sushi. You could always order online from Amazon or Jet to be more discreet.

You can use pretty much any rice, it's just that some types will work better than others. What's more important is the preparation of the rice itself. Using sushi rice vinegar or sugar and salt + fanning the rice properly will make it really sticky. There are Onigiri kits that actually keep the seaweed separate until you want to eat it, which is awesome. You might want to look into those too.

Good luck!