If you download a map from OpenStreetMap or Google Maps as an image, you can use it in Tableau offline.
And it is not actually that difficult. It is well described in Tableu documentation. The key thing is that you need coordinates at the edges of the image, so Tableau can position the image accordingly.
Give Tableau the image via Map > Background Images. You'll need to provide the map edges' coordinates.
Switch off normal maps (which don't work offline anyway) like this: Map > Background Maps > None.
Display generated latitude and longitude in the view.
Now you probably still don't see a thing. You need to adjust both y and x axis range, i.e. set it to the the map edges' coordinates.
That's it.