What type is that glass? I own a Magic Build 3D Printer. I found out, that the glass is scratched like in the picture.
So, I'm looking for a replacement glass plane to put in front of the projector. Is this a special glass? What are the properties? Where can I find it?
This is a sheet of glass you'd most likely need to acquire as a replacement part for your printer's manufacturer:

The dimensions of the glass, especially with the rounded corners, are usually very odd and make buying a spare from a glassmaker really expensive.
Some printers like the Elegoo Mars have an LCD mounted as part of this sheet.
Printers also might have this plate coated for different properties, for example, to reduce reflectivity and refraction inside the printer.

In this case, we have a DLP setup that uses a projector. You best inquire at the manufacturer itself using their website, E-Mail or facebook about a few spare pieces - that will get you the same quality as the bed you already had (save for the scratches) and they usually have such items in stock.