You can get the tiles from openstreetmap.org, or from bing.com, or whatever. The easiest way would be to connect N900 to cheap-and-quick Internet, and ask Modrana to download the tiles (around a place or a route; between appropriate zoom levels). As far as I remember, Modrana doesn't delete tiles, so anything you have downloaded beforehand will be easily usable once you are offline.
I don't use Modrana much now; experimenting with Marble. Not sure which one of them is better.
Best wishes.