Check out Numrich and Sarco.  Also, buy a couple month's worth of Shotgun News, as the vendors for such stuff rotate their ads, and the stuff in them.  It's out there, be patient.  Reply above was also good advice.

If you google "Arisaka Stocks" or "Arisaka Parts", and follow the leads, eventually you will break into the Arisaka parts folks, and then you will be GTG.

Once you've done that, you have a choice to make.  You can restore your Arisaka to original, as-issued condition with all the parts made by the correct vendors and so forth, or you can elect to repair it with parts made by other vendors, some of which may be reproduction parts.

You will need to be familiar with the markings on your Arisaka, and what they mean.  Google arisaka markings and go there first.  IIRC, North Cape books probably has an affordable book on Arisakas, which discusses the markings.

Your rifle will be worth MUCH more if restored with correct parts, notwithstanding the parts will cost more.

However, you must be honest, and tell a prospective buyer that your rifle is restored, as opposed to an original.

If you elect to simply repair the rifle, any collector will detect the mis-match of parts.  for that matter, some collectors are pretty good at sniffing out restored rifles.

If money is not a major issue, I'd restore it, as your rifle has an intact "Mum", and is worth restoring.  Besides, the correct parts to do so are getting scarcer as time goes by.  A correct restoration stock might cost a few bux today, and be unobtainable a few years from now.  YMMV.