URGENT! Mama cat needed ASAP

My sister rescued 2 kittens who were very young and 1 of them is thriving, the other (we believe to be the runt) is not doing so well! He stopped bottle feeding so he went to the vet today and he had IV fluids to help make him stable, but he really needs a mama cat until he’s ready to be weaned because he is refusing a bottle.
Per the vet, he has no fleas or worms or illnesses so he isn’t contagious to other kittens. He just needs to be fed! Please let me know if you have a mama cat than can foster my sister’s kitten until they’re ready to be weaned. She is wanting to keep the kitten and wants him back after he is weaned! Thank you!

She is in the 78717 (Avery Ranch area of Austin)