I recently purchased a Piranha Evo marker. I also got a revolution hopper and it doesn't fit. Please tell me how to make it fit or what I have to buy.

I don't know about the revolution hopper, but I know the halo and a few others you have to sand the neck on the hopper down to fix the gun, this may be what you have to do. If you don't want to do that you'll need to get an adjustable feed neck for the gun, it has a allen wrench on it to tighen up the hopper.
If you don't have a store near you. Look online at paint ball central, they'll have everything you need.