There is a non-official source package for git-annex in [AUR](https://aur.archlinux.org/packages.php?ID=44272). You can then build it yourself or use a wrapper for AUR such as yaourt:
$ yaourt -Sy git-annex
---- I'm told the AUR has some dependency problems currently. If it doesn't work, you can just use cabal:
pacman -S git rsync curl wget gnupg openssh cabal-install
cabal update
cabal install git-annex --bindir=$HOME/bin