You can’t have a Schreibstube, without paper.
I collect many different types of wartime paper items including Feldpost letters, identity documents, passes, ration and personal kit item packaging and other ephemera from the war years. Paper used during the war came in a wide variety of textures, colors, and thicknesses but there are characteristics that I associate with wartime paper in general. Materials shortages necessitated the use of even things like potato stalks in German paper, it was different from most paper commonly used today. Most commercially available modern paper is made differently from what was used in the first half of the twentieth century, in general. Modern paper is brighter and smoother whereas wartime paper was often rougher, with a slightly mottled appearance. Of course there were exceptions to this, smooth paper did exist back then.
Having a Schreibstube necessitates having a variety of different kinds of paper for different documents and purposes. I have found it very difficult to source exact modern equivalents for wartime paper, but I have been able to amass a lot of usable paper that looks close enough to fall generally within the very wide range of what was used. To reiterate, you are looking for paper that is not bright white, and that has some texture. One thing I do from time to time is go to an art supply store and look at sketch pads. There are a lot of different types of these pads available and many have usable paper. I will also look at stationery and try to find nice paper with subtle watermarks, or without watermarks, or various kinds of natural colored or off-white stock for specific projects.
One thing you can keep an eye out for is paper with a high recycled content that has not been bleached to bright white. Paper and envelopes in “Recyclinggrau” color, that may be available in Germany, look great.
If you look at pads of “kraft” paper, or colored “construction paper,” you may find usable stuff for making envelopes, small passes or ID documents, or other specific purposes. Here in the US, dollar stores often have sketch pads with thin newsprint type paper that is great.
Newsprint in general is very good to have and is widely available in pads. A common issue with this is that it doesn’t stand up to fountain pen ink, which will bleed and feather. So newsprint alone is not suitable for all purposes.There is paper that is marketed today specifically for fountain pen use.
A lot of these pads of paper are 9” x 12” which is ideal, as you can cut that down to the DIN A4 size that is and was standard in Germany.
Lastly, keep an eye out for actual vintage paper. In antique shops and thrift stores you can sometimes find old typewriter paper or old notebooks. Even small notebooks can yield great paper for typewritten Soldbuch inserts or other small documents. And old books may have blank end pages that can be utilized.