A zero would be the smallest. We just bought a one, but I think she is a two. Be careful of unscrupulous people trying to sell starved puppies, they aren't Chihuahuas lol, I see that around here, its sad. I go by the size of there paws, and by the teeth. I have a five pounder from Canada. He's ferocious, I think here he would be a one or so. Anyone is going to lie and say that it is going to be smaller than it is, just because the smaller they are the more money. The parents are a good indicator, but a litter can have zeros ones and twos.
We got ours from the vet near here in bavaro. Our fully grown dog here was six months when we got him and he was smaller than the girl we just bought.
Prices range from 100 to whatever they think you will pay.
Boys also seem to be smaller.
Good luck. You dhouldnt have a hard time finding one. The challenge is getting a zero.