Until more complete information can be posted here, Barbara
Barton's "Gardening By Mail" makes a wonderful jumping-off point
to plant societies who have seed exchanges or give-aways, rare
seed sources, plant/seed finding directories. Some societies can
be located on various webpages along with the current membership
info, usually a negligible cost to join.
or try:
The Virtual Garden
E-MAIL LISTS:
The Seeds-List
Alpine-L
The Mediterranean Plants List
SEVERAL FAVORITE MERCHANTS:
Companion
Plants
James L. Hudson, Seedsman/ Ethnobotanical Catalog of Seeds
(Highly recommended as a source, the calalog is indispensible as
a souce of rarer information, honest perspectives and updates
about some of the issues surrounding conservation) Heronswood
Nursery
SOCIETIES Etc: (A few; Also look for regional chapters)
The North American Rock Garden Society
The Hardy Plant Society
The Hardy Plant Society, UK Rock Garden
Club Prague
Alpine Garden Society
Herb Society of America
Royal Horticultural Society
Alpine Garden Club of B.C.
The Flower and Herb Exchange
The Permaculture Seed Exchange
Chronos Apollonios' "Home on Olympus" 
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