You can use any of the formulated substitute milks - Cimicat is one. Many people use goats' milk. You can get Lactol and tiny feeding kits from the big pet shops and I'd hazard a guess it's all much cheaper than going to the vet! Rearing rejected kits is a thankless task though - very few survive to adulthood. Much better to find someone with a foster mum if at all possible. Rabbits will foster kittens very readily and successfully.
Sue