Where can I buy bay doors for 1958 4104?
International Bus Parts (IBP) in Florida would be a primary source. Sorry, I don't have the contact info in front of me at the moment, but if you do a web search or search past threads on this board they should be pretty easy to find. I believe they recently released a new catalog on CD.
One other detail to mention involves the color matching. The aluminum panels are anodized, and you may see some variation in color from one door to the next. It is sort of the nature of the beast. No matter where you purchase the replacement(s) you can expect that it may not be an exact color match. Or course if you intend to paint it something other than it's natural color, it becomes a non-issue.
Deans Coach in Durham, NC.
U.S.Coach in Berlin, NJ.
There is also a website geared toward 4104's exclusively, http://www.pd4104.com/ (http://www.pd4104.com/)
They might be able to help you find them.
IHTH
Dallas