I'm rather new here and will admit that I haven't had time to fully read everything on the site so I am not sure if this has been posted before or not.  I have had some fantastic luck over the past 2 years in finding plants in my area through Craigslist.  Go to the site, choose your area and search for plants.  Broad terms such as plants or perinneals will give you greater results but you can also search for individual plants as well. 

This past week I went to 2 peoples houses and dug up daylillies, hosta, liriope, iris etc.  This past weekend, after dividing the plants I had over 250 plants...full size plants, not just tiny specimins that will take years to mature.  Oh, and did I mention that I spent a total of $20?  I am going back to both places this week and get even more.  If you can't find anything the first time, try, try, try again!!! 

Last year I got liriope, vinca, daylillies, narcissus and 2 HUGE bridal wreath spirea bushes for a total of $40.  Also, if you are trying to find something in particular or want to make a little money selling your own plants, post an ad for free...you never know who will respond.  Good Luck!