They're a random drop. I have about 20 sets at this point and have found them all over, including a set when I first got to Sanctuary in a locked suitcase.

Easiest way is just to buy them in Diamond City.

I also found 4 sets yesterday at the East Boston Prep while I was there w/Strong.

And I personally haven't seen this vendor sell them, but I don't doubt Tarelyn at all. When vendors sell rare items, like a certain somebody that sells certain hard to find pieces, the save-reload-look-reload-look-reload thing works but you have to be patient. It once took me almost two hours of reloading to find something I was looking for. I was this close to ranting in the spoiler section about how much I dislike the fact that finding something could be so hard if you weren't too chicken to go exploring at early levels.

Summer shorts are rare? Really? I should open my own shop.

I found them!! yes, now i can have leg and arm armor and look all innocent as i murder raiders by the masses

I found one in Home Plate, in one of the pre-built containers.