What to feed for a 3 week old puppy? Mother won't let him suck anymore.

You want to get a formula such as this: PetAg Esbilac Puppy Milk Replacer at PETCO

For up to 6 weeks.

In addition to feeding the milk replacer do you know when you need to start feeding solids, and when?
It doesn't sound like you did much research before your dog had puppies.

It's a puppy of my neighbor. Their dog gave birth to 6 puppies and she's letting me adopt 2. I just took 1.

I had 2 dogs before this one, they both died this year so I don't want to make the same mistakes I had.

It really is a good idea to take the puppy to your vet, let him/her recommend a diet.

The issue of feeding isn't the only problem. At 3 weeks of age you pup is not ready to leave it's mother, for more reasons then just food.

You're looking at health problems and behavior problems (if the pup survives and the odds aren't great).

The fact that you really don't know what to do scares me. Caring for a 3 week old puppy is not something that most people can do successfully.

I would suggest going to your local library and picking up some books on raising puppies, to be honest there is far too much information to learn than can be posted here.

I love this site from Woodhaven Labs, there's lots of info on dogs and breeding to get you started but there's nothing better than having a hardcopy book sitting there for reference.
Breeding Responsibly - Finding a responsible breeder - Bringing up puppy

And here is a site on orphan puppy care:
Orphan Puppy & Kitten Care - VeterinaryPartner.com - a VIN company!

Thank you all...