A map is not really a hardware defined concept in anything since we stopped calling them game screens, give or take a few minor technicalities for some games if programmed a certain way.
Anyway that is not really all that helpful. As Mikemk said you are going to have to either rip and stitch, or you are going to have to make a level viewer for yourself. The level editor approach is learn how to edit levels and then make a program that decodes and views the one you have in the game, not beyond anyone really but not something you can learn in a few minutes either.
The simple way is to get an emulator like no$gba debug or desmume with a tile/BG viewer and use that. You could also try screenshots if you can disable certain layers or sprites or something.
I have not got things to hand so I will have to link
http://problemkaputt.de/nds-tv1.gif as it will be one of the things you see. With the exception of things with a level editor or if the devs released their own maps then anything you see for a full map which is not a screenshot of an in game feature will probably be this.
For the DS the trouble will come with games being 3d a lot. You think New Super Mario Brothers is a nice 2d game a la classic mario? Wrong, full 3d, just a locked viewpoint that is side on. You can still get much done with that but it might be harder.
