My husband took our tub faucet apart to check why we were not getting hot water. He took the part to Home Depot to ask them about it. They said it is an instore part and they do not stock it?? Why HE (the clerk) said to buy the whole fauce?? We cannot take our whole tub apart to replace a single little part PLEASE HELP::::

]My husband took our tub faucet apart to check why we were not getting hot water. He took the part to Home Depot to ask them about it. They said it is an instore part and they do not stock it?? Why HE (the clerk) said to buy the whole fauce?? We cannot take our whole tub apart to replace this tiny little part for the temperature. PLEASE HELP