You should be able to find the BIND source by visiting http://www.isc.org/bind.html. You need the bind-src.tar.gz package. Be sure to get the latest version!
Things can get a bit confusing at this point, because different parts of the BIND package will be referring to the same directories by different names (depending on whether or not they're running inside the jail). I'll try not to confuse you too much :-).
The main directory that we have to worry about here is /var/run, because its contents are required for both the main named daemon (inside the jail), and the ndc utility (on the outside). We'll start by setting everything up to find this directory from the outside world. To do this, we need to modify src/port/linux/Makefile.set (substitute your port's directory if you're not running Linux), and change the line
toDESTRUN=/var/run
While you're in there, you may want to change the other destination paths fromDESTRUN=/chroot/named/var/run
/usr to /usr/local.
Now everything should be able to find that directory... except the named daemon itself, to which it's still just /var/run inside the jail. We can get around this by making a small change in the named source. In the file src/bin/named/named.h, find the line
and add the following line immediately after it#include "pathnames.h"
This way,#define _PATH_NDCSOCK "/var/run/ndc"
named will ignore our definition of DESTRUN over in Makefile.set and use the correct location (from its perspective in the chroot jail). You will notice some warnings about redefinitions of _PATH_NDCSOCK when you do the build; just ignore them.
You should now be able to compile BIND as normal, following the instructions in the INSTALL file. At this stage, we only want to compile BIND, not install it. Don't go too far when following the INSTALL file. Essentially, it's just make clean, make depend, and make.