how to get source files for nios gnupro tools kit

You need to use the gnu source that was compiled for the version of Nios you are using.

Since there were no changes between 3.1 and 3.2, you can use the 3.2 source.

Microtronix has a linux kit for Nios I, but it is not free. See their website (they host this forum).

Email me khallbec@altera.com and I'll send you the ftp site for the source.