Who can foster a puppy for a family, puppy belongs to an 11 yo girl, please help?

I have been babysittng a friends' puppy and they are unable to take him back after I have had him for over 2 months. He is beautiful, a cream/white fox terrier chihuahua mix named Rico. 7 months, still teething, pad trained and loves cats. He is very bright. I am a 60 year old with my own problems and can no longer provide for Rico. His other mommy is devastated but cannot take him until they find a new home for themselves. I believe they could pay a fee but not a regular fee, due to having to save up for a new home. Please, if you can help, the very last thing I would have to do is to take him to the Humane Society, and as you know, there is no room at that Inn. HELP!