You should be able to get the sjock rebuilt here... http://www.mickgardnerracing.com/motorbikes.html
.. for around the £30 mark, they have done them in the past.
Risse Racing (USA) do a nice range of shocks if you were thinking of upgrading, here... http://www.risseracing.com/proflex.shtml
, they do occasionally come up for sale used on eBay Uk, so keep an eye out for them.
Noleen J6 still do the OEM shocks as fas as I know, here... http://shop.noleenj6.com/category.sc;jsessionid=6602651F3C14D6B559060FD4837DA8C0.qscstrfrnt06?categoryId=7
You could try and get an adapter made up to let you run a more modern air-shock (I've PM'd you regarding this
), seen a few different variations, the one I had worked well and used a Fox Float shock.
Used Noleen shocks do come up on eBay and on here now & then as well, so that's another option.
Replacing the rear shock seems to be the biggest bugbear on these old Proflex bikes, on older models the elastomers disintegrate, and on later models the shocks become tired and break down, things aren't helped by the design that Proflex used ie conical mount at the bottom/bolt through at the top as no modern shocks will fit the frame without an adapter being fabricated.
I found that a modern air-shock and air forks transformed my Proflex into an (almost) modern short travel XC machine. If you're not bothered about "period correct" etc then it's well worth looking into if you intend to keep the bike.