Ouch... The supply line is behind the drywall behind the tub if there is is a wall behind the tub that you can get to... If not then it's in the wall itself (I assume an outside wall)... In that wall is a divertor valve that mixes the hot and cold and goes to the tub/shower... In order to get to it you have to cut into the wall...

Most of these have a cut-off valve under the house between the meter and the valve... The problem here is that it will be copper and souldered... Now you are in over your head...

Back to trying to fix from the outside...

B~