Whilst they are probably the most difficult piece of a camera to make (with the possible exception of the bellows) they are not that difficult. Getting the right type of spring steel is probably the difficult bit.
Whilst getting a pre-made back is the easy option, I think it's better to have built all of the camera yourself. However - I did buy a spring back for my first camera!
I have a drawing of a spring back in AutoCAD format. Send me a PM with your e-mail address if you would like it.
Steve.