For gaskets and seals, try Napa. But if you need hard parts, try
Northwester Transmission Parts. They deal in vintage transmissions. I have dealt with them personally, very good service.
4 Speeds by Darell might be another...n with Darell as well, very knowlegable. Just from the description, sounds like a detent has gone sour, whcih will make it jump out of gear. The speedo is most likely an unreleated problem, could be a stripped speedo drive gear, plastic gears are not uncommon in thise, or a bad cable.
Your trans should be an SM318.
