There is a non-official source package for git-annex in AUR.

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 gpg openssh cabal-install
cabal install git-annex --bindir=$HOME/bin