Ratrescue.com - Message board where you can find/post rats for adoption as well as coordinate and help out with transports!
PetFinder - Search for homeless rats in hundreds of shelters across the nation. There are rats frequently for adoption.
Also, give your local animal shelter a call. Not all shelters are listed on PetFinder! You can check with local veterinary clinics and search the internet for local shelters that may have teeny furries in need of forever homes!