They are filled by pressurizing then welding the pressure fitting shut while pressure is applied. Sounds like fun......
They can be "rebuilt" by re-pressurizing, but I don't imagine it would cost much less IF you could find someone to do it for you.
The nitrogen eventually diffuses through the diaphram in use, it rarely actually peforates. However, with all the flexing it does in use, re-pressurizing is probably only a short term fix anyway.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!