Try reading this section:
Yorkie Rescues & New Homes Needed - YorkieTalk.com Forums - Yorkshire Terrier Community
There is a list of Rescue groups there.
Fostering is not easy. A lot of the rescue dogs have issues. All of the rescues I have fostered have been rescues from puppy mills.
Read all the info in the rescue section and make sure this is what you really want to do.
Foster homes are always needed.
Thank you for considering fostering a rescue.
The 501(c) 3 is a charitable designation - donations are tax deductable (I believe that is what it means)