good question! havnt had this before.
Normaly you show the dealer your gunlicense with a blaser rifle in it. then you can buy other barrels suitable for the model you have.
I think you dont have a license for your gun, its a question of bureaucracy. the dealer have a gun business book with all guns and guns parts under law like replacement barrels in stock. If he sold one, he write it in the book with the name and adress and the license number of the customer.
Maybe you cant get in your hands in germany, you can buy it as a foreign citizen but it will be send to your adress per mail.
best is you contakt big dealer like frankonia for this or the german custom

you find here a e-mail adress, think its no problem if you make your inquiry in english