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
