The ones I made are from a Burda pattern, under the label Burda 7381. http://sewing.patternreview.com/patterns/43801 I recommend it if you have about medium-ish sewing skills, but even if you’re a beginner, the only thing you might have a bit of trouble with is the mock-fly. They came out looking like this!
My biggest tip is to get shorts that are cut for/made for men/boys! Regardless of if you’re a male or female cosplayer, I think that getting shorts with the right shape are really important to the overall look of the cosplay.
OldNavy.com has a pair for $15 here: http://oldnavy.gap.com/browse/product.do?cid=41566&vid=1&pid=520702042
I’m also about 90% positive that Target also has a similar pair in their men’s AND boys section right now. If you’re unfamiliar with men’s and boys sizes in the US (as I don’t know where you are) men’s sizing goes by your measurements (so I have a 28 waist, I wear a 28. 34 waist is a size 34, and so on,) and boys sizing is a bit trickier, but an 18 is about the same as a 28, so a 16 is, I assume, the same as a 26.