Roland Should still be able to obtain spares for the JP from Japan,i had to order some Potentiometers for mine last year,and the guy said they can get most parts,but it'll usually take anything up to 12 weeks..
I would say you should be able to still obtain parts for most synths of the last 15-20 years from Roland or Korg and Yamaha ,alot of the more common components are usually cross bred into other synths,things like Keybeds and Pitchbenders or knobs dials etc,if the synth is more exclusively made in smaller numbers then you'd probably have to try and source either a broken machine or have some sort of special Order by the manufacturer which will be considerably more expensive,than an off the shelf part
I think it depends on the manufacturer(the more succesful the name brand the more chance of spares)and how commercially well the synth did when it was released,the more popular the synth was the more likelyhood of spares availibility now,I have had no problem whatsoever obtaining spares from Korg so far back as the M1,so that pretty much almost 20 years ago and most of the components are availible for the M1 and virtually every other synth since 88 as there would have been a larger scale production of the components,on more popular instruments so spares would also be ordered as a backup,and excess components usually would be put into a stock situation for future use..
I'd try your local roland Distributor and ask to see if they can get the parts,they usually have a computer system with exploded diagrams and can pinpoint the exact Part reference numbers and chack stock availibilty etc
