Here's an article with recommendations for Atlanta, as well as lots of pics:

Here is a link that might be useful: Peonies in the South

Sorry to report that the company featured in the linked article, is no longer in business. The property where the garden was located, was in an area zoned for industry and was sold for development several years ago. The many existing Chinese Tree Paeonies were moved, but the business never reopened.

I grow a couple of the Chinese one's, which were purchased bare root from HD many years ago, but I often feel that it's an exercise in futility! Seems that each year, when the beautiful, but fragile petals unfold, the weather becomes very uncooperative. Gusty winds and brief, torrential downpours strip the flowers unmercifully. I have become accustomed to a very brief period of enjoying the flower show!
Best advice! Purchase from a reputable company that offers a wide range of colors and specializes in Tree Paeonies, to better insure a quality product. Be prepared for sticker shock! Insure that all planting and care instructions are followed.
Sit back and enjoy and hope that the weather is cooperative!

i would think that most cultivars would be suitable for your zone 7. i have a couple in zone 8 and they are doing o.k. but not as vigorous as i would like (likely the summer heat or lack of winter chill). i have them sited where they get a few hours of direct sun. ordered them from van bourgondien nursery, about $30 each. to continue what the above poster mentions, i have actually placed protection over mine during a storm when they were blooming to protect the flowers from being pounded and shattered! they certainly have big flowers and are quite spectacular.
'shima nishiki' {{gwi:362227}} {{gwi:362226}} and 'kamata nishiki' {{gwi:362228}}

Here is a link that might be useful: nursery

The source shouldn't be the problem. There are many good reputable sellers out there. (Always check Garden Watchdog before placing an order, and, see the link below, BTW!)

Whether certain varieties perform well in Atlanta may be an issue. That's what the link above was about. Although Alpharetta lists zone 7, I'm wondering if that is correct (maybe a microclimate situation?). These days, Atlanta is probably best described as zone 8b.

Here is a link that might be useful: Bad News!