Jerven themselves states that the the army has used the Fjellduken about 20 years, not 30 years.
The reason there are no surplus ones is that they are primarely issued to specialists and there are not that many of them (not many soldiers at all nowadays). "Ordinary" soldiers may buy them directly from Jerven a little cheaper than civilians get them, but these obviously never hit the surplus market.
I know that the I-force in Heimevernet was issued these some years ago, and there has been rumors about issuing them to all soldiers, but I don`t know if the have begun this yet (I`m "retired" from service and don`t really care anymore).