4 Answers

 Take it to the vet and let them nurse it until it can survive on its own.

There is puppy formula available in most pet stores and small nursing bottles.  Phone your nearest pet store and phone your Vet for advise... you will be able to purchase those items at your Vet's office as well.  This is not uncommon.

As everyone has said call your local vet there are puppy milk and bottles out there but I am more concerned why mom has rejected her pup there could be something wrong with it so you must get this checked out asap as mom knows best always if there is nothing wrong with the pup you have a long hall ahead every 1/2 hours you will have to his/she mom feed and do the motions by stimulation on the rear end I have reared up 10  kittens this way and its very tirering do take this puppy to the vet first and get it checked out good luck

You can get mllk things and tiny bottles from your vet. However mother dogs will reject a pup if they fell something is wrong with the pup.