Ah, you want a development kit (see
http://www.devkitpro.org/). Obtaining the source code will be next to impossible. Even if you do manage to get ahold of a copy (it will be highly illegal), you still have to understand the programming language to make useful changes to it. And still, the original game was written for the official NDS SDK, which is illegal software for you to have; it will not compile with devkitpro any way.
Creating a half-decent disassembly (note: this is not "source code") can be done with IDA Pro (costs ~$3,000 USD) but it is not perfect, and will at times mistranslate a lot of the data. Also, there will be few points of reference for what each subroutine does, since function names will mostly be unavailable, and zero source code comments will be available. Even still, you must be very familiar with the ARM and Thumb instruction sets, and assembly language programming in general, to do any hacking in this way.
In other words, it is not very possible to do what you want. Having a decent NDS debugger (none exist yet) is probably the best bet for such hacks.