The port libstdcxx does exist.
$ port info libstdcxx
libstdcxx @4.7.2, Revision 2 (lang)
Variants: (+)universal
Description: The GNU compiler collection, including front ends for C, C++, Objective-C, Objective-C++, Fortran and Java.
Homepage: http://gcc.gnu.org/
Library Dependencies: gmp, mpfr, libiconv, libmpc, ppl, cloog
Runtime Dependencies: gcc_select, ld64, cctools
Conflicts with: libstdcxx-devel
Platforms: darwin
License: GPL-3
Maintainers: mww@macports.org, openmaintainer@macports.org
Please run "sudo port selfupdate" then try again.