I believe you can 'retro fit' a regular baseball cap to appear this way. Its a tricky process and can be painful but the results are fantastic and you cannot tell the difference from the naturally occuring backward hat.
I know there is a black market for rear facing hats which i believe is where most young people obtain theirs from but i cannot condone this as you often see botched operations with hats ending at a jaunty angle and sometimes seemingly just placed on the top of the head.
I know the government are trying to tighten up the legislation on this but many say it is unenforcable.
Have you tried ebay?