Ttyjukl

Sirius in Prisoner of Azkaban mentions that Wormtail intentionally sought after a wizarding family to take him in so that he could keep an ear to the ground regarding any news of his old master Voldy.

We know that Wormtail belonged to Percy before he was Ron's, but I cannot find any canon stating how he came to be with the Weasleys. There are a few possibilities:

• Wormtail Apparated close to the Burrow, turned into a rat and then what? Just waited around for the good-natured Weasleys to find him and take him in? I don't see it as being a logical jump.


• The other possibility is that he was taken in to the Magical Menagerie and the Weasley's just happened to buy him, rather than an owl or other magical creature that was allowed at Hogwarts. It could have been any wizarding family and just convenient that it was the Weasleys.

Both seem to be far fetched, but possible. is there any canon stating how he came to be with the Weasleys?
from Prisoner of Azkaban - Lupin says that Wormtail had been part of the Weasleys' family for 12 years, i.e. within months of his disappearing act, how?

We don't know

So far as I can tell, there is no definitive information about how Peter Pettigrew ended up with the Weasley family.

That said, what little evidence we do have leans toward your first hypothesis.

We know he was seeking out a wizard family, specifically. So at least he was not found by mere coincidence:

"Why else did you find a wizard family to take you in? Keeping an ear
out for news, weren't YOU, Peter? Just in case your old protector
regained strength, and it was safe to rejoin him...."

—Harry Potter and the Prisoner of Azkaban

Would he have been taken in by a magical pet shop? It seems unlikely. Recall the dismissive attitude that the witch at the Magical Menagerie had to a rat of his type:

"An ordinary common or garden rat like this can't be expected to live
longer than three years or so," said the witch. "Now, if you were
looking for something a bit more hard-wearing, you might like one of
these --"

She indicated the black rats, who promptly started skipping again. Ron
muttered, "Show-offs."

—Harry Potter and the Prisoner of Azkaban

It seems that the sort of rats sold in magical pet shops (of which there must be only one or two in all of Britain) are of a somewhat more fantastical variety. It seems unlikely that a shop would waste time catching a common rat, rather than breeding their own.

The Magical Menagerie employee referred to Scabbers as a "common" or "garden rat." It seems most likely that that's exactly where the Weasleys found him: in their garden. It might seem unlikely for the Weasleys to adopt him, but why not? Witches and wizards have a tradition of keeping rats as pets, much more so than Muggles, it would seem (in keeping with folkloric ideas about witches' familiars). If a friendly stray dog kept coming to a Muggle family's front door, they might well take it in. I think the Weasleys would have done the same with a friendly rat.

The Weasleys aren't that well off, he was probably at a magical pet shop, marked as a common rat at a low price. What wizardry family would buy a common rat as their pet? A poor family, the Weasleys. Either that or it happened that he may have been the last animal available for the Weasleys to choose. Until J.K.Rowling confirms anything it's left floating in the air, so what ever is logical I suppose.

I know I'm years late to this thread but I don't think Scabbers would risk going to a pet shop. Owls eat rodents. In the books the generic "owl pellet" is used but I would bet these are mostly meat. A pet shop owner who finds a "common garden rat" isn't putting it out for sale, he's feeding it to the more exotic creatures. Peter is a coward. My guess is that he just wanted a warm place to sleep where muggles wouldn't poison the food and house elfs wouldn't chase him out. Befriending the five year old must have been unintentional because have you ever watched a five year old play with a rat? Yaowch! Preschool pets all have this thosand yard stare that let's you know they have seen some shit. I don't care how well behaved Percy was at school, at some point he squeezed/dropped/trod on that rat.

I think Peter didn't have a master plan to be a spy. He might have said that to Voldemort but only after Sirius accused him of being a spy and he realized it was a convenient excuse to keep a trigger happy Dark Lord at bay.

TLDR: It was all an accident. Don't give the rat too much credit.

It said Percy bought the rat his first year at Hogwarts 1987. Peter Pettigrew had been Scabbers since 1981 when Sirius tried to kill him on a Muggle filled street. He cut off his own finger, so that it looked like he had exploded, and then he turned into the rat.

He probably went back to Hogwarts for safety because he could hear rumors, gossip, etc there about Voldemort. While there, he probably found his way into Percy's bag or something and Percy kept him.