Get the snapshot archive via FTP from <URL:ftp://sources.redhat.com/pub/libstdc++/>. You will almost certainly need to use one of the mirror sites due to the extremely heavy load on the main server.
The master CVS repository for libstdc++-v3 is now a part of the gcc repository. As such, checking out the gcc module includes the current libstdc++-v3 development sources. To check out libstdc++-v3 only, use libstdc++-v3 instead of gcc as the module.
Once you have the GCC sources checked out over CVS, you can use the contrib/gcc_update script to retrieve the latest changes, and automatically update the timestamps of certain files. With the timestamps updated, you won't need autoconf or the other tools (unless you make changes, obviously).
You can also browse the CVS repository over the web.