Anna Jameson at Brite Leaf kindly replied to my inquiry and also provided some useful information from Dr Bowman. I quote:
"I apologize for the delay in responding. We were out sick and quarantined for Covid.
From your list, US 897 is the only one we grow. However, I do not have any available rootstocks. All have been grafted and are contracted to other customers. You might try Southern Citrus Nursery in Dundee. (863) 439-3694 They used to have a website, but I don’t wee one now.
Also please see notes below in red regarding the other rootstocks. I’m attaching a guide that Dr. Bowman provided for your reference. -- according to Dr. Bowman (the USDA plant breeder of all these varieties)
US 801 Changsha x Eng.Sm. TF: not released to the industry; don’t know anything about it
US 809 Changsha x Eng. Lg. TF: not released to the industry; don’t know anything about it
US852 Changsha mandarin x English Large trifoliate orange: not an industry standard in FL; the only ones I’ve grown were from a seed source in GA that has now been cut down
US-1279 Changsha’ mandarin x ‘Gotha Road #6’ trifoliate orange: this doesn’t come true to type from seed, so must be grown as tissue culture or rooted cuttings – we’ve [ Dr Bowman ] had no orders, so are not growing " -- Anna Jameson
Well it is good to know. I have the idea that using industry root stock is going down the wrong path. The industry needs apomictic varieties and cold hardy plant breeders want the opposite. In the time it would take to get the bud wood, and grow it up I could perhaps produce a changsha X PT cross and grow it out as well. It may be few years behind but then it would probably be more zygotic than anything that the industry would release. By the way Bright Leaf has a great selection of reasonably priced citrus. I glad to have been introduced to them and plan to put in an order this spring.