Makaton.org has some free resources, but it is a bit limited. In the past there have been some sites around with signs, but Makaton is copyrighted, so such sites are probably in breach of copyrights and thus they don't seem to exist for long.
changing soon best place is local library for books. Also links nannynick has posted are very handy as is mr tumble. I also used this when I started to learn a while ago which I thought was a nice introduction: intro video
I found flash cards really really helpful too and if you have time you can make them yourself using the Mr Tuble site and some laminating stuff - either a machine or self laminating pouches.