I found this thread on diyaudio discussing the same question, there are several other related threads; I think the summary of what I've been reading would be: find a reputable vendor on e-bay, and maybe query a purchase with some-one on diyaudio who has done first to avoid being ripped off, or follow up one of the (many) links in those threads to see if an equivalent part actually exists for such an old technology, which from a quick skim read I have some doubts, as from what I just read a depletion FET now is in no way the same as the obsolete parts you need.
Hopefully some-one else can guide you to an existing equivalent part. Good luck!