Googling on "Prunus spinosa US" gives this
where in the USA can you get black thorn berries (sloe berries)?
which seems to answer a lot of your questions, including a supplier in Oregon, which would save any problems over restrictions on import of plant material (I suppose it may have been you who asked the question in the first place!)

Here, I would plant young bare-rooted (ie supplied without a big ball of soil and a container) plants about 12 to 18 inches high in the dormant season, ie October to March. I'd expect 95+% of them to take without any further attention.

But - I've always planted them on a moisture-retentive clay soil. On a soil which becomes dry in the summer, they would benefit from watering during their first summer. I don't know whether you have any small mammals which like to chew bark - you might need to provide protection.