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.