Don't get a "feeder" rat. That just increases the demand for them, and you aren't doing any good "rescuing" a rat that probably had a bad, unsocialized, unhealthy start in life. You are just supporting the breeders that treat rats as a food crop for reptiles.
Go to a breeder, or if you must "rescue" something try the local animal shelters - they often have rats as people get pregnant rats from pet stores, or accidentally have a litter, or have to move a long distance and can't take the rat, etc.
Also - if you are getting rats with strange health problems, and tendency's to bite you aren't giving your money to the right people. Find a reputable breeder, or a pet store that handles the rats, and gets them from a reliable source that supplies them for PETS, and not food. A good indication I've noticed is the price - all rats are cheap, but "Pet" rats are usually up around $10, and feeders are more like $2 - $3. The difference is how they are bred, maintained, and handled prior to sale, and the slightly higher price will prevent most reptile owners from buying them for food. The good stores will make you sign a form stating that you will not be using them for food, and will guarantee the rat for at least 15 days. The store I use gladly took back a sick rat that I brought in (only had him for 3 days), treated it for 3 weeks 8O , and then offered him to me again in good health. That's a sign of a GOOD pet store - willing to provide no-cost medical care to backup a warrantee on a rat that *might* make them $5 in profit. They clearly lost money on the animal, but customer service, humane treatment of the animal, and repeat business was more important.
Rick