Gas Hot Water ???

I have a gas hot water heater in the house and the pilot went out this makes the seonc time in two days. I re light and it stays on and get the water hot and then about morning time it goes out again. I was told by a retired plumber he thought it might be the Thermo Coupler I believe that is what he cause it. He said it wasnt hard to put in and fairly cheap, he said if this didnt fix it then replace heater. First I was just wanting to know the corret procedure of this and can this be bought at Lowes or HD and what would it cost about

If you look at the thermocouple it will unscrew from the valve and pull out from the holder next to the burner. Then reverse the procedure with the new one.

How old is the heater?

Yes the big box stores sell them.