Over the years of keeping a step van running for our shop I've ordered lots of items from Mill Supply (www.millsupply.com (http://www.millsupply.com))
I've recently found that while they are not a bus parts supplier per se, many of the parts they carry have been working really well for my 4106. Switches, wiper parts, driver fans, etc. They also carry a large range of heater parts and controls which I've used in many 'off label' ways in the past.
Just a suggestion for anyone needing another parts source.
Richard