A clean install of 4.3, obtained from /pub/OpenBSD/4.3/<arch>/ at your nearest mirror would install "4.3-release." You will need the matching ports tree, obtained from the same mirror, in /pub/OpenBSD/4.3/ports.tar.gz.
The fastest way to prepare the build is to install all dependent packages needed to build Postfix, first. Setting your FLAVOR environment variable, then issuing "make print-build-depends" will give you the list of packages to install. Set your PKG_PATH appropriately to the /pub/OpenBSD/4.3/packages/<arch>/ of your nearest mirror.