Where can I find the latest version of the SI4T code that can be used for T2013? I want to create my own TBB to extend the index data.
Do I need to use this branch? https://github.com/SI4T/SI4T/tree/develop
The TBBs do not (yet) use any API features introduced since Tridion 2011 so the latest source code (in the develop branch you link to) will compile against all versions of 2011, 2013 and SDL Web 8.x
You may have seen it already, but this article has an example on extending the indexing behaviour by reusing the existing TemplateBase and SearchData classes.