The responsible way to buy boxer puppies is by going through a breeder who is registered with the ANKC or any other proper authority and one who follows all of the RSPCA guidelines.
This way, you can avoid scammers and have peace of mind knowing you've made the right choice.
You should always verify the breeders by contacting the authorities that they are registered with. Only go through official websites, and know how to look out for sketchy scammers. Doing as much research as possible is always a good idea.
Only buy puppies or even adult dogs from reputable and registered breeders; this way, you know that you are getting a dog that has been well cared for, is happy and is in good health. Always do background checks on the breeder, and plan to meet them in person.