I've seen a link on here to a directory of tartan background images. Coincidentally, I make my own, seamlessly tileable tartan background images. They are seamlessly tileable right down to the pixel, and they are accurate to within two threads of the historic threadcounts - in some cases accurate to every last thread. The reason not all of them are historically accurate to the last thread is because I use the Twill Pattern which was available on Windows 3, and it requires threadcounts to be equally divisible by four in order to tile seamlessly, so, in cases where the threadcount is not equally divisible by four, I add or substract one or two threads to make it so.
Anyway, I just added a Danish and a Scandinavian Tartan wallpaper to the collection I'm building at: