A few questions about FreeBSDWhere/how should I obtain the source code (ex. through terminal, Download off website)How (on ubuntu) should I build it?Before I build it can I customize it (in other words is possible)?
FreeBSD source and how to build
kernel;freebsd;compiling
You can check the source for FreeBSD out of version control here.  The developer's handbook answers a lot of questions about developing FreeBSD.  Why aren't you building it from FreeBSD itself?  It seems kind of... odd to be building from Ubuntu.