I know this sounds like a weird suggestion, but check Craigslist or Kijiji (or similar sites depending where you're located) for rats that need new homes. My two newest rescues are boys from Craigslist. No parasites, no abnormal aggression, just boys who got dumped on someone who wasn't prepared for rats. In fact, one of those two Craigslist boys is currently sitting on my shoulder, bruxing and boggling away just because he's sitting on me. Also, check Petfinder (or similar site) under small and furry animals. You can find rescues that way.
While this a rat rescue forum, and we will always push for rescue over purchasing, we also understand you may not have any rescues around you. If that's the case, please consider bringing home at least two babies.
I love baby rats just as much as any other rat lover, but bringing home homeless rats is a special feeling
And, at least for me, I've had an easier time bonding to and introducing older rats than babies. Babies don't have the confidence that adults do, even if they're currently homeless. Sure, some rescues come to us with quirks, but the best part is bringing a shy rat out of their shell. It's doable. Of my three original rats, one named Amun was a shy little guy that blossomed into my most social rat, right behind his brother, Gideon.
Parasites and diseases are always the biggest worry when bringing a new rat home, but stress from losing their buddy or sibling can be enough to make them sick with no contact to other rats. Just like humans, chronic depression or anxiety lowers their immune system and makes them more susceptible to illnesses.