I got the springs from a local guy the works on a lot of Ford tractors, he had them in stock. Any New Holland dealer (they bought Ford) will have them or can get them. Or Goggle the part # I listed and you can get them from most any after market Ford tractor parts suppler.
The local truck shop had bigger detent springs from an Eaton tranny that I think would work for the transfer case but I decided to use my old ones as it will be rarely shifted.