Boxer magazines publish ads from breeders. Usually any time after 7-8 weeks.
Many states have regulations regarding minimum age.
However, your individual circumstances must be considered. Remember also that the puppy under 3 months needs lots of time to rest. While much can be told at six to eight weeks about the puppy's eventual looks, if you are seeking a show prospect you might be better off waiting until the pup is six to eight months of age. At that time, be prepared to pay a higher price for all the additional time and effort that the breeder has spent raising the show potential Boxer. This choice is a matter of personal preference.