Nobody's going to sell you a high powered laser, that would be a lot like breaking the law. You need to get the parts (legal) & learn to build one yourself (also legal)
However.... Is there a waiver, contract or something that exists protecting whomever that might agree to sell a 'buildable' kit for selling to someone? If something stated that the purchaser (1) knew of the danger that comes with an HPL and (2) must exercise the same level of caution as one would with a loaded firearm ...followed by a few disclaimers and that strongly emphasizes on SAFETY (?)
I don't know, I just woke up... I need my irish coffee. I'm pretty sure something like this is already out there though, it just needs to be brought to the surface