KM -- replied, and thanks.
Merlin1952 I'd say apart from wanted forums like on here, the best bet is old bike shops or coops, or ebay. As suggested in my original post, the springs from the different VX series models seem to be interchangeable, at least. Also, I'll be making sure from now on to run this derailleur with the longest chain I can possibly give it, to avoid too much stress on the spring.
cheers