I believe there was only one release of the Celtic Thunder and it has been available for purchase from the start.
Because there is a sub-set of people in the rocket community that prize "collectibles", Jim will hand number some of the first kits of each design (maybe 100). You'll have to figure if there is anything special about keeping it in the original packaging or building and flying it (building and flying is more fun). Check eBay to see if anyone is getting outrageous prices for their higher numbered kits.
BTW, my Celtic Thunder is #65.