wheezy-backports contains packages from Debian testing, recompiled for Debian wheezy. These packages are called backports. They are not part of the default distribution, they are provided as optional extras for users who want newer version of some software. If you have a wheezy system, add backports to your list of packages sources:
deb http://http.debian.net/debian wheezy-backports main
/etc/apt/sources.list.d/backports.list, then run
apt-get update then
apt-get install exfat-fuse.
You can also download a package directly and install it with
dpkg -i FILENAME.deb. This is not recommended because you will not get any automatic security upgrades for it, and you'll need to take care of any dependency manually.