I doubt a puppy is the way to go. Most dogs past the age of 3 don't care for puppy biting games. Over Thanksgiving I had my 12 year old Lab, her mother, a 12 year old male Golden, and our 9 week old Lab, Nita. The Golden did play some with Nita, but the 2 Labs wanted nothing to do with her sharp teeth. The 3 older dogs have visited all their lives, but didn't seem to play much together. Maybe I should have seen if they enjoyed competing at fetch, they used to.
Unless your Boxers were more playful than most older dogs, I would look at at least a 3 year old. There are many nice older dogs needing a home, some of them even registered pure breds. Try
www.petfinder.com or
http://www.akc.org/breeds/rescue.cfm
It is hard to say if you and your other dog are ready for another dog. My Aster and her mother didn't seem pay much attention to each other, but may have enjoyed being together. In the past, they did play together long after they quit playing much with other dogs. Your dog could resent a stranger coming into the house too. A neutered male might be the best choice.