Are you looking to purchase peonies or just to see peonies in bloom? If you are interested in seeing peonies in bloom, I heard the following places are good to check out:

Scott Arboretum of Swarthmore College
NY Botanical Garden
A Peony Garden, in Glen Mills, PA - Check our this website to find out more about this garden: http://rootstoblooms.com/tag/a-peony-garden/

This year the APS sponsors their annual peony event at Longwood Garden. To find out more, check out their website: http://www.americanpeonysociety.org/events/2013-convention

Have fun and maybe I will see you at one of these events.

Karen

I would be more interested in harvesting fresh seeds in the fall as I grow peonies from seedling every year.

Do any of these places let people harvest tree peony seeds?

I really do not know, Stevelau. But you can always email them and ask if they offer them. I heard the NY Botanical and Swarthmore have many peony trees. I have never been there myself but I am going this spring as my interest in peonies only began last year.