One of the best derailleurs ever made! I'm not aware of spares
ever being widely available though, except for jockey wheels.
You may get lucky, eventually, but in the meantime your best bet must surely be to look for a complete derailleur, on
ebay or at a cycle jumble.
Maeda made virtually identical units badged for Raleigh, which you might find offered more cheaply - not quite the
cachet of Suntour!
Lists of cycle jumbles
here and with almost total overlap,
here and
here. (yet other lists may be found).
to the forum - been a while between joining and your first post, I see. Don't be shy; we're mostly harmless.
PS: might be worth repeating your request in the Vintage & Classic section - some of those old farts rarely venture as far as Mechanics & Repairs, and if anyone knows about old and rare spares, someone on there will.