I recommend the first section of Vegas 1 here. You'll want to start off the combo by hitting the two balloons that float extremely closely together (touching) and then hit another 3 that will be spawned beneath them. If you screw up, you can just jump onto the next platform, time it out and hit 5 consective balloons without a problem. Video below:
Another quite easy place to do this is on London 4, near the end there will be a few sections of spawning balloons that you should be able to jump on, back and forth and they will continue to respawn.