I am wondering because I may want one, especially considering its the only true citrus that will grow where I am, notwithstanding hybrids. I would want at least a 2 year old tree, with a good set of foliage (preferably not from seed)
this is an outfit here in the Bary Area that specialized in more unusual Asian stuff:
South Pacific Orchid Inc
Here is a link that might be useful: South Pacific Orchid Inc
Woodlander's nursery in Aiken, SC has it listed on their website.
Taiwanica wont' survive in z7 without protection. If you have to protect anyway, why not plant something edible like a satsuma. Citrus hardiness to 15F doesn't mean 24+ hours of freeze. 24+ hours of freeze kills all citrus, hardy or otherwise.