Housing with rats can be a touchy subject. Although there is no scientific data to back it up, the recommended spacial requirements for pet rats is 2.5 cubic feet per rat. Here are some cages we recommend.
Midwest Critter Nation/Double Critter Nation
Can fit up to 6 rats for a single and 12 rats for a double
Hagen Living World Ferret Habitat
Can fit up to 3 rats
Kaytee Multi Level Exotic Pet Home
Can fit up to 3 rats
You can also make a DIY bin cage for much cheaper.