I bought my first hickory board from Timberline in Kent, don't think they do them any more. Not sure where I got my second. I know I hand picked the first. I wanted it for making a self bow, so all one piece of wood, no laminating. Had a hell of a job finding a flat-ringed board. I know now this wasn't necessary for the bow (fibre parallel to back, but ring orientation less important). The second board was less flat ringed than the first and the explanation was that the boards are imported for bowyers, who being English make "traditional" bows by laminating hickory with various tropical hard woods
and almost invariably turn their hickory into 1 to 1.5 inch wide, 1/8 to 1/4 inch strips, and ring orientation isn't something they care about much. So finding a board that is thick enough for an axe, and has grain in the classic "ideal" orientation could be a lost cause, or at least a costly one.
If you haven't carved an axe handle (or similar tool handle) before, ever mind being a dab hand at hanging them, I would recommend ash over hickory. Hickory is nicer to work than ash in that the early wood in ash makes getting smooth faces difficult in some places, but it would really suck to find a rare bit of wood and make a beginner's goof. I did okay with the last two ash chop tool handles I did, but my hammer handle before that could be better, and while my parang handle is fine now, it is the third iteration. That is with all the tools, and a fair bit of practice with knife handles.
Best of luck! Do show pictures when it is done