I'd suggest getting the Thunder B. The player package is $33 (total of 13 shells). You just need to get 12g Co2 cartridges, and possibly have to put a penny at the bottom of the cylinder the cartridge goes into, so the needle pierces the top of the cartridge every time. Test your grenade before you do so, as my friends may have been a defective one. If you put the penny in first, and yours didn't need it, you may not be able to pull it out.
It's a damn good grenade, too. Definitely gets peoples attention, and will scare you, if you're not expecting it. This warning is one my friend learned: warn people before the game what it is, and to not pick it up, until after it's gone off, or they might have to change their pants.
~Brian