The easiest way would be by seed. B&T World has some, but you order and then they go out and get the seeds. They also have a germination guide.
Buying trees by mail is a bit of a problem because of weight and size, so you may end up paying more for shipping than the cost of the tree. Do a search online for Nyssa sinensis uk and that will get you a couple of nurseries you can contact.
Also, be aware that Nyssa like water, and with the level dropping in the reservoirs, it may be a bit of a problem in the summer when they are growing.

If you can't find any have you thought of Liquidamber instead?