I assume that you are interested in propagating from seed?

I also suggest that you visit the Annuals Forum and talk with Zen_man. He's been breeding zinnias for many years and posting his results in a series of fascinating installations. He always extremely happy to share information.

Select Seeds specializes in heirloom plants, but I do not believe that all of the plants they sell are heirlooms. They offer several mums, and although I don't really know about all of them, I am pretty sure that Emperor of China is not patented.

After seeing your post I decided to look for more information about heirloom mums. Perhaps you are already familiar with this blog post, but here it is in case you haven't seen it before. This post has links to other posts about heirloom mums and also recommends King's Mums as a good place to purchase them. It looks like King's is already nearly sold out for the 2020 season, and some of their varieties have been removed from their website. Since King's specializes in growing mums, I would think they would be a good source of information regarding which varieties are not patented. Bluestone Perrenials also sells a wide variety of mums.

I've had lots of fun looking at all the different types of mums there are - a much greater variety than I ever knew existed. Thank you.

Thanks, guys. After going down the internet rabbit hole, I connected with some chrysanthemum enthusiasts who grow them for competitions and exhibitions. They almost exclusively use non-patented mums. Turns out, there's only 2 nurseries in the US that specialize in heirloom mums - King's Mums is one of them and happens to be the mum society's main source - which would explain why they're nearly sold out. Anyway, at least I've identified a source. I'll just have to put my order in extra early next season. Thanks for your help, folks!

@Plantloverkat - I won't say that the patented mums we see in all the retail stores in the fall aren't lovely - they are - but, wow! Some of the heirlooms are stunning and so diverse. I feel like we've all been missing out.