I've heard that there is the mod, that makes everything that trader could possibly have in stock available all the time, but I couldn't find any links. Maybe someone know, is there really such mod, where it can be downloaded and will it work with gog version of Arcanum?

I remember this one. As stated in the readme, rename one of the files to InvenSource and place it in the Arcanum\data\rules folder. If you don't have the rules folder, create it.

http://www.terra-arcanum.com/downloads/arcanum/doa/invensource.zip