Well, the JT Splatmaster
line includes a couple of different targets. A lot of this stuff is available at Wal-Mart. If you want interactive targets, a lot of what is available at sporting goods and outdoor stores like Dick's or Gander Mountain would work just fine with paintball. Probably not the exploding targets, though, unfortunately.
Really for paintball it's more about snap shooting than bench rest target practice accuracy. How fast can you go from crouched behind a bunker to marker out and on target? Especially if you're playing Xball or speedball in general. So set up drills for yourself where you break from cover and time how long it takes you to put any ball on target, even if it's not the first ball out of your marker.
Now if you're trying to play stock class or even open class pump it's a different story.
But yeah, drills and practice are probably more important that what exactly you use as a target.
I like the tennis ball idea, though. I may have to steal that