Puppy formula.. I've hand raised newborn baby squirrels. They are more closely related to dogs than cats. They do very well on liquid puppy formula fed with a tiny feeding bottle.
Most important thing.. Mothers clean the babies rear end so it can go potty, otherwise they can get blocked up and die. If it's young enough to be bottle fed, you have to gently wipe it's butt every day with a warm, wet, cloth to keep the area clean and stimulate it to pass waste.
They are a lot of fun but when they are eating on their own and running around climbing everything in site, you will have to release it. They make terrible house pets. Put a dish of food outside for it or on a tree squirrel feeder so it has food while it transitions to being outside.