Hi Buckshot,
From your Jaycar reference I assume you live in Oz.
A few years back I also needed a spring, and searched Yellow pages for "spring" factories in my neck of the woods. The place I found didn't have exactly the same thing, but 2-3 different sizes with approx. the same diameter/length/gauge, and with a bit of experimentation, tweaking of the spring and the little swivel anchor point, got it to work satisfactorily.
John While Springs
12 Larkin Street
Riverwood NSW 2210
Phone: 02 9533 5366
If this isn't near you, you'll have to do some legwork and visit some spring manufacturers.
If you need belts, I had obtained them from Wagner Electronics, Ashfield NSW.
Some years ago I also needed to get the pinch roller fixed up. I found Terry Witt (t.e.witt@juno.com) in the USA, sent him my roller and he rebuilt it to brand new state.
Regarding 1/4" audio tape, you might try:- Allans Music, Billy Hyde's, DIck Smith, Tandy, or audio places.
Again, hope this helps, kind regards, bruce.