I've never seen tree peony seeds sold at stores before in packets, but I get mine off of eBay where I can get pretty good deals on lots of seeds. What I find is that fresh seeds that haven't dried out that were recently harvested ie, when I go to my neighbors yard and take their seed pods to grow them, they tend to sprout the following spring. Now when I buy them from somewhere, they tend to let the seeds dry, and then the germination usually takes a little longer so I may end up getting leaves either during the summer or it may be another year in the spring. Other than that TP seeds generally have a good germination rate 50% or higher depending on how fresh the seeds are.
The way I plant them is simple. I simple take a screw driver and make 2 inch deep holes to drop each individual seed a few inches away from each other so when they sprout, it's easier to transplant them in the fall once they go dormant. If you want to try growing tree peonies for yourself, I would recommend this seller or any of the other ones that sell a variety of seeds, in the same packet so you don't end up with all the same color.
Free shipping 100 Chinese Tree peony seeds(mixed color) | eBay