Oklahoma city is 7a, there are too many tree peonies fits your requirements, you can buy from any site to sell tree peonies.

Most Japanese tree peonies and parts of Chinese, America and French tree peonies are upright, and semi-double or double.

There are several online sellers that have a nice variety of tree peonies, take a look at http://www.treepeonygarden.com/english/products/products-eng.htm , https://www.treepeony.com/ , or http://www.solarisfarms.com/ Those are Tree Peony Garden, Cricket Hill and Solaris Farms. They seem to have good reputations, I ordered from Solaris and Tree Peony Garden for the first time this year myself so can't give a personal recommendation but have seen several forum members say they were good. One more is Chateau CharMarron, http://4peonies.com/ I ordered herbaceous peonies from them last year and was very happy, saw pictures of tree peonies others ordered and they looked very nice for the price. Good luck!