C130,
Try this link:
http://www.adventurerv.net/
I am not affiliated with them in any way, but have ordered parts from them in the past. To find the water heater door, go to "major appliances" then click on "water heaters" and scroll down, I think the door you're looking for is on the second page.
You'll find almost anything you could need on their website. I often find that it's cheaper to go to the local RV dealer for most things since shipping costs far more than the 6% sales tax and pricing is almost the same online and in the local store.
Anyway, I hope this website at least gets you started.