I have difficulty giving advice on nursery-grown bald cypress. I get all of mine for free. That said, if it were me, I'd follow the same protocols with a large nursery BC as I would a swamp-dug tree. Wait until a time in January/February just before the buds break. Then take it out the pot, cut the root ball down, chop the trunk, and remove all branches below the chop. Submerge it for the first year.
The amount of roots I leave on a BC is determined by the depth of the growing tubs. If I were to do something similar to a tree in a 45-gallon pot, I'd have to check the distribution of roots first. Nurseries up-pot their trees by putting the tree into a larger pot and topping it off with more soil. This is okay if you're eventually dropping the tree into a landscape. However, it may mean that the original surface roots are deep in the pot. Further, as the BC is potted into deeper and deeper pots, new root growth will sprout from the trunk. It'll do so with an uneven distribution. Just chopping the root ball could be the worst thing to do. Be sure to check the first few inches of the trunk below the soil line before you buy the tree.
I've never shipped trees. If I did ship one to Texas, I'd have to get a fire-ant certificate because parts of Texas are still outside the fire ant's distribution area. The closest I get to Dallas is Shreveport. When the Louisiana Day of Bonsai is in Shreveport, I go up there with some trees and slabs and a couple dozen 5-gallon bags of bonsai soil. That would be about a 3-hour drive for you, though.
You can contact Charles Canfield or Jane Hall with the Shreveport Bonsai Society to see if there is anyone they know of who sells bald cypress for bonsai.
