Manufacturers and Commercial Sites
This site has all sorts of industrial supplies. You can probably build just about anything from this site. I got bungs for the 55 gal drums and the shower stall from this site.
Guardian RV Generators by Generac
This is the official site of Guardian RV Generators by Generac. I have a 5500 watt Guardian powering my bus. I have been very happy with the generator.
Here is where I got the foam for my custom-sized beds. I did all of the ordering online and they shipped me the custom sized foam and custom-made moisture resistant fabric covers for the foam. This was a great investment. I am pretty sure these guys will ship almost everywhere.
I spent so much money at home depot during the creation of my bus project, they should probably be linking to my site. But seriously, there happens to be a huge home depot just two miles from my house so when I needed something for the bus I usually went there first.
This is the company that makes my driver's seat. It is an electric 6-way with a swivel. It is very comfortable and sturdy.
This is the manufacturer of the mirrors that I have on the bus. These are super cool.
I got a lot of the RV specific parts for my bus from these guys.
Forums, Clubs & Individuals
BruinGilda the AeroSwine
This site was one of the first sites I used to get inspiration for my bus. I was amazed at what you could do with a bus if you tried.
If you are doing an American school bus conversion, this is the site to check for great information and other people who are doing the same thing.
UK Self-build Motor Caravanners Club (SBMCC)
I recently found out about this site that originates in the United Kingdom. This site has a wealth of information and tons of photos of projects including a London double-decker bus conversion.
Okay, this site won't win any design awards and some of the material is outdated. Still, it's a great place to find bus conversion resources and links to tons of conversions.