That looks like a Noleen NR4 shock with the piggyback resevoir, you should be able to have it serviced at any MTB shock servicing company eg TFTuned for example, or a motorcycle shock servicing company should be able to help, Noleen/J6 have a USA based website where they still stock these shocks and parts too.
My rear shock is also on its way out, I've been trying to look into having an adapter made to run a more common shock such as a Fox Float, which also has a bit more adjustment on it as well, other people have had adapters made so it's just a case of measuring and then having it made up.
Risse also do replacement Proflex shocks, but the cost of these or a replacement Noleen unit has put me off buying one as yet.