Water leaking inbetween wall

On our most recent weekend trip, we decided to use the shower to fill the toilet (we had to replace the shower attachment due to that was previously leaking) and my hubby yelled for me to turn it off. He said that there was water running down from between the wall. This only happens when the tub/shower faucets are turned on. This weekend he opened up the area surrounding the faucet (have to remove the caulking and screws and panels) and found what appears to be a gray 1/2" weird shaped gasket looking like it is cracked on one end. It is where the hose connects to the hot & cold faucet. Does this sound familiar to anyone? If so where did you find the replacement gasket?