Motors for tennis ball launcher?

I have about two weeks to come up with a tennis ball launching mechanism at the very least and I'm just kinda of stuck on what to use for the motors, preferably something that doesn't need to be geared up for speed.

My school has a power supply that they can use, so power isn't a problem. Does anyone know anything that I can salvage for a cheap price? And if so, where abouts can I get two of those in Melbourne?

Edit: Forgot to mention, I'll be using the two wheel with the ball feeding through the middle design. I'd like to make something that can launch it at least 30 meters but I'm not sure if that's feasible with how much time I have.

Perhaps some numbers? - wheel size, wheel mass. And what about the trajectory for the 30m range? Max range is when launched about 45 degrees to the horizontal, but that's unlikely for a tennis ball launcher - so what actual ball velocity do you want?