I have Boxers too and back in the dark ages when they had their pups we used the good ole newspaper. Craig's list is probably better now...
Anyhow - please, please please screen your prospective families using some kind of questionnaire form and vet check. Require them to sign a spay/neuter agreement! I hated to think of my baby's babies out there being bred... (I tend to think of the worst in these situations). Use Legacy Boxer Rescue as a resource www.savetheboxers.org they have a great application that you can print off and have people complete.
Let me know if you have questions or need help - I'd LOVE to see pics of your furry grandbabies!!! ____@____.com
My 10 year old girl is hanging in there, but has inoperable Mast cell tumor and at some point we'll add to our Boxer family, but now is not the time. =(