I have bought frozen rats from two different places, Rodent Pro and American Rodent Supply.
Rodent Pro has a table at the All Ohio Reptile Show and I bought rats from them one time. I was very unhappy with the quality of the rats, they were all covered in urine and feces. Another thing I was disappointed in is the fact that their prices at the show are much higher than the listed prices online.
When it came time to get more rats I decided to order from American Rodent Supply and I was VERY happy with with them. I placed my order on a Saturday night, the order shipped on Monday and I had my rats on Tuesday. They came in a large box, very well packed with three blocks of dry ice. The small rats were vacuum sealed while the large rats were in Ziploc bags. The rats themselves were all very clean.
American Rodent Supply is more expensive than Rodent Pro's online prices but the quality of the rats more than make up for it, plus they offer free shipping to many states.