You can download it and build it yourself from http://gcc.gnu.org
. You should install it in a separate directory, e.g. /opt/gcc-4.3, so as not to mess with the existing gcc on your system. You should *not* attempt to replace your existing gcc, just install the older version in a separate directory and set your PATH and LD_LIBRARY_PATH appropriately when you want to use it.
What do you need such an old version for, anyway?