You can either order them preregistered from ebay or your local slightly shady phone resale store. It's also likely possible (though i havent tried) to use eSim and global providers like airalo.com to get one without enforced controlled registration (unlikely that they check, specially if you register via proxy).

And lastly, especially if you live in west german you can just take a train to the netherlands and by a sim there, no registration required.