I was given a old montgomery wards mitre box (no. 84-3975m)but no back saw came with it. So I need to get a saw, but I do not know where to find it.

Take the box to any of the big box lumber/hardware stores like Home Depot or Lowe's. You should be able to find a saw that fits snuggly in the box. You don't have to get a Wards original replacement part, fat chance of ever doing that.

I have the same box and saw

I have the same mitre box with saw woundering how old it is?