Here are some links. I think we need to expand our search to the Chevy City Express and to the NV200 taxi. I e-mailed some guy in England that does conversions.
This one looks like you can build a window any size you want:
http://www.motionwindows.com/products/
I talked to this guy. He custom makes windows. Has done for sprinters and Ford transits. Optimistic we can get whatever you want.
http://www.rvwindows.com/campervan-windows/
Here is a guy doing research on a forum:
http://nissannvforum.com/topic/721-nv200-camper-windows/?hl=windows
Get our size right and we might be able to find something on eBay
http://www.ebay.com/bhp/rv-sliding-windows
UK - I e-mailed them
http://leisurevehiclewindows.co.uk/
A window maker in Indiana
http://easyrvwindows.com/rvwindows.html
About $215 with my estimated size. We can get it exact
http://easyrvwindows.com/showprice.asp
teardrop windows and parts. Some to choose from:
http://teardroptrailerparts.com/Windows___Doors.html
Here are vents and fans.
http://teardroptrailerparts.com/Vents.html
SLIDING WINDOWS. Did you get an answer for this? I am doing the same research. I have a 2015 NV200. There are many conversions in Europe. I know of only Recon in USA. I think he has nice sliding windows, but I can't afford a full conversion. I found this place in Washington that will custom make windows for you. What about the Taxi windows? Who makes those? http://www.motionwindows.com/products/