Some of the old DeCarbon style shocks used a resealable bung in the reservoir that you inserted a hypodermic needle into for gas filling. You should find a phillips head screw in the middle of the reservoir. Remove this to expose the bung. Pressurise to 175psi. Removing the seal head can be a problem if you're unfamiliar with the design. Yell out if you need help.
I would enquire about it with those guys first. When I was at RT in LA they told me how much they charge to service those shocks. I was amazed! Apparently no one wants to touch them. They have their tricks but they are by no means difficult. RT have a range of parts also. So supply shouldn't be a problem.