NVidia don't release source code for their driver. They don't even release complete hardware documentation to allow others to write open-source drivers for their hardware. I think that they are afraid that their competitors would learn too much about the internal workings of their cards if they could get hold of such code or documentation.
However there is an open-source driver for NVidia cards which doesn't support 3D-acceleration. It is included with recent versions of XFree86, and is called "nv". This is probably your best bet.