The short answer - no, i don't know of where to pick up one of those - i looked in the past myself, and i didn't find anything - in hind-sight of an experiment, i'm glad that i didn't - it would have been a waste of money, with the range issue already mentioned.
What i did find, were similar rockets to the ones that the nerf launcher would use at the Dollarama. I picked up two to do an experiment. I shaved them down to the diameter of a M203 launcher, and used a paintball M203 shell as the launcher charge.
Results - 20 foot, maximum. Not worth the effort.