The Victorian group of the Australasian Native Orchid Society is the
organisation Dave's talking about. Unfortunately we can only distribute
tubers to members within Australia. We have enough problems getting
tubers through interstate customs so a sending them across
international borders is not feasible.

If none of the US sources the others have mentioned pan out, Kevin
Western (in South Australia) does sell flasks of terrestrials.
Email/phone him for a list of available speices. His contact details
are on his website. http://www.arcomnet.net.au/~western/index.html
I find Pterostylis deflask very easily. mycorrhizal association, I
deflask into a relatively sterile mix of perlite and commercial potting
mix so, unless the orchids are forming associations with Legionella, I
don't believe that not having the correct fungi in you potting mix will
be a problem with prolific growers, such as Pterostylis nutans. Diuris
can be a little more touchy. Whether this is due to fungi or just a
general dislike of deflasking I don't know.

Andrew