The only one i know if is the saftey devices ones.  These are/were very expensive and also I heared that they dont make them anymore (unconfirmed).  May be worth talking to them direct for a dealer.
I am after one as well and have looked for a generic ladder as I imagined that there would be loads that bolt on to the rear tyre as so may 4x4 have rear mounted wheels.  However I have never seen one for sale, in a catlogue, on ebay or even on a personal site.  Therefore Its not looking good.  I was even considering making one out of a bunk bed ladder.
Anyone else any ideas.