I think I am correct in saying that you cannot get them re-gassed. I think it would have to go back to the original manufacturer.
I think the simplest idea would be to buy some new ones. If you look here :-
http://uk.farnell.com/
and do a search for "GAS SPRINGS" you should hopefully find what you are looking for. If I recall correctly mine cost about £40 +VAT for the pair.
Hope this helps!