Your best bet would probably be to contact the national (or regional) boxer breed clubs in countries that interest you. Just as in the US, they are probably best placed to put you in touch with responsible breeders.
It is worth being aware, however, that you're unlikely to encounter terribly many good breeders falling over themselves to export show quality puppies to the US. The reasons are quite understandable if you think about it: for one thing, responsible breeders usually want to place pups in homes where they can keep something of an eye on them and require that they be returned if (for any reason, or at any time) the new owner cannot keep them. That's not possible to achieve if you ship a puppy half way round the world. And a second reason is that when show breeders place show quality puppies, they usually wish to be able to access those pups in future for their own breeding programs. And that's likewise not possible if you ship to another country. Logical enough - who spends years developing a line and then sends the future generations out of reach? In short, there are significant negatives from the breeder's point of view. And no significant positives.
That's not to say you'll never find someone willing to ship a show quality pup. Just that it probably won't be that easy. And that you shouldn't be offended by it - the reasons are good ones
Do beware of brokers, especially if the pups they're offering come from puppy-mill capitals like Poland or Hungary.
Another thing to bear in mind if you're looking to import is that in much of the world, including quite a few countries in Europe, docking dogs is illegal. From those places, it is impossible for you to get a docked puppy. And currently near impossible in the US to successfully show an undocked dog. Your choice of European country to import from is thus limited if you want to be showing the pup in the US.
Incidentally, you won't get an AKC or UKC registered dog anywhere in Europe (unless it's an American dog imported there, of course). Neither the AKC nor UKC exist outside of the US. The registry used by most countries round the world is the FCI - Fédération Cynologique Internationale or World Canine Organisation. That includes most countries in Europe, though some, such as the UK have their own separate national registries. Any dog registered with a recognised national registry, however, and that is properly exported with an export certificate, should be registerable with the AKC.