I would Bing the internet (I use bing, but you might use Google or some other search engine) for "flared-leg gored shorts pattern" or some variation of that -- maybe "wide leg" -- you get the idea.  Then you will just have to sift through all the results,  most of which will not apply to your interest.  I did that for fun and to test my theory a few minutes ago, and I got a plelthora of information.  I sifted through some small portion of the information and didn't find EXACTLY what you're looking for, but enough to think there is useful information if you look hard enough.  I believe most of those type shorts patterns will be vintage -- as in the 1940's.  Now, that said, you could use a plain short or a split skirt/culotte pattern and modify it. 

Good luck.