In the years I've been around Sprinters, I've only heard of Motion Windows (a.k.a. Peninsular Glass in Vancouver) making easy-to-source opening replacement windows for the T1N (2002-06) North American Sprinters.
The German company Seitz makes appropriately-sized dual pane acrylic windows for the cutouts in the Sprinter doors, but they are not readily available to individuals in the US.
If you do not yet have a window in the spot where you want one, you can obviously cut a hole inside the stamping and use a wide variety of windows. Because the T1N stampings are rectangles, rectangular windows smaller than the stampings don't look too bad. The situation is worse on the newer NCV3s, where the stampings are a parallelogram.
Here's a photo of my first Sprinter, which had the Motion replacement rear windows and Seitz side windows a bit smaller than the cutout stampings: