You should be able to download or get a link to sources in the same location you downloaded them from originally.
For instance these threads:
Distributions
includes links to the source code of the engine.
If you downloaded from somewhere else, you need to find out which version you got. Keep in mind some sources comes from the private subversion which is only for premium member. If that's the case you have to get the sources elsewhere or preferably get an official distribution and start working with that.