It's in the libstdc++-v3 directory in the g++ (gcc-c++) sources.

You shouldn't have to manually download this library, if you're on Ubuntu Linux, it should be shipped inside this package :

sudo apt-get install libstdc++6

If you already have libstdc++6, then the problem is elsewhere and you should explain what you're trying to achieve in the first place.

If you want a newer version of libstdc++6 than the one provided in the default package, then you can try to update to the toolchain test package :

sudo add-apt-repository ppa:ubuntu-toolchain-r/test 
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

Otherwise you would have to compile GCC from source :

• Install the prerequisite (using sudo apt-get build-dep gcc-4.7 as instance)
• Get the source from GNU.org
• Compile it using configure, make and make install