Do you have anyone near you that raises birds ? Anything from ducks, quails, chickens, turkeys, or even parrots or budgies ? Or an SPCA that has a dept for birds ?
A feed store may be able to sell you chick food, but often it is a 50lb bag, so not a great format when you don't know if it will eat it.
I guess something like mealworms might be easiest to find in a small bag and I think all birds will welcome a bug snack. (local pet food store)
Please no sugar water, plain water in a bowl you place below the beak and raise the bowl to dip the tip of the beak in water. It will swallow what gets in the mouth, just don't go too fast, it can be worse to have too much water too fast if it is dehydrated.
If you have any poultry farms nearby it might be time to meet your neighbors (bring a smile and pictures of the baby, but don't drag h with you, we all worry about bird illnesses being brought into the house where it could infect our birds)
Thank you for trying, but don't scold Mouse... She has her nature too...