Yes it is possible to legally obtain them. BUT most of the ones offered for sale are most likely illegally collected. It is still legal to colect them in WV, KY and maybe PA, but you might need a fishing license.
These salamanders are only suitable for advanced hobbyists who want to try and breed them, but not suitable as pets. They require specific care and an aquarium of no less than 55 gallons. I suggest that anyone interested in keeping hellbenders, either collect your own or don't keep them. Otherwise you are supporting the commercial trade in a species that cannot handle commercial collection at this point.