draw it out yourself. if you can build it, you should be able to plan it. worst case is to take the plans for a 14xX and just extend the long side out to your desired length.

84 Lumber used to have plans for playhouses, sheds and the like. Maybe they still do and you could modify a set for your dimensions.

Im sure there's more out there, just search engine shed/workshop plans

Here is a link that might be useful: shed plans