I know there's one in the first Mason Dixon Knitting book (check your library).
How about
:
http://www.fiddlesticksknitting.com/...avan_robe.html
Couldn't find any free ones, but you should also look at cardigans, like this one, could easily be knit longer and called a robe:
http://www.bernat.com//pattern.php?PID=2353
Were you looking for adult or child size?