Big box store (Sears, Home Depot, Lowes,...)
Haven't needed to yet
By the way, when I bought my water heater from HD I had originally contracted with them to put in my water heater since I didnít think I knew how to do it and I was afraid of blowing up my house if I did the gas connection wrong. But since they were coming out the next day to replace it I drained my old leaking heater and pulled it out of the way so I could get behind it and do some work. When the plumber sent out by HD arrived with my new water heater and saw Iíd already pulled out the old one he asked me if I wanted to put the new one in myself. I said I didnít know how. He showed me how it was simply a matter of putting the new one in where the old one was and putting everything back like it was before I took the old one out. He even gave me a couple fittings. He was very courteous and very helpful. (I think he was glad to have one less water heater to put in. It was the 4th of July weekend, it was very hot, and he had about 11 water heaters to install that day). And he was a professional plumber, not some fly-by-night operator.
Iíve read more than a few posts where people slam HD. That is unfortunate. Take them for what they are Ė a home improvement supermarket. They do have some people who are knowledgeable. When I needed to put in a drain tile system to keep the water from pouring in my basement the man there helped me pick out all the parts I needed. Our local HD is closing and I am very sorry to see them go. There is another one 15 miles away so it will not be a quick run to get what I need the next time I find Iím short a part.