Try this site, it contains technical and programming information on the Gameboy and Gameboy advance consoles. There is a lot of programming information which I understand may be relevant to the Nintendo DS.Jeff Frohwein's Software/Hardware Dev'rs
It is an old site with a lot of dead links. The information by the site owner is quite interesting. You may be able to pick up a second-hand Gameboy for next to nothing. I have second-hand one that was given to me, unfortunately I am useless at programming.
I found some old information about a guy who used a Gameboy for a KAP pan and tilt controller. Unfortunately the technical information has disappeared
The above link has some port mapping software available. Maybe it may be of use to the creative programmers on the forum.