libtoolize is part of GNU libtool, a package for building libraries portably. On the Mac, one option for getting it is to use MacPorts, a package manager which works in a similar fashion as Gentoo and FreeBSD, in that is compiles packages on your machine. See http://www.macports.org/install.php.
Beware, though, that it will be installed as glibtoolize, i.e. with a 'g' prefixed. That is a standard way to make GNU tools live in parallel with UNIX tools of the same name, that might be present (even though there isn't one in this particular case).
The command for installing libtool from MacPorts is: sudo port install libtool
Add a '-d' flag after the 'port' command to see the build output.