Low Cost/No Cost
Plant Exchanges
These are nonprofit organizations dedicated to preserving rare and endangered plant species.  The members share seeds for free or a nominal fee.  I have no personal experience with plant exchanges.
Locate plant exchanges by contacting plant societies for individual species (i.e., Historic Iris Preservation Society) or by searching the WWW for "plant exchange."
Plant Societies
There are societies for many different plants (including roses, irises, and orchids).  Members may be willing to sell or share plants from their own gardens.  Also, societies sometimes hold plant sales.
Find out if there are local chapters.  Yahoo is a good place to start a search  for plant societies. 
Friends who garden may be willing to share.  Some plants multiply until they are too crowded and need to be thinned out.  This is your opportunity to take the extras.
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