Just find them. There are 3 general stores on each map and at least one won't show up until you're within a few feet of it.
I have found all 3 in the Frontier and in Boston, but the one in North Boston still isnt open for convoys. with the others, they were available as soon as I walked in. glitch maybe?
Have you liberated the area it's in? Only thing I can think of...maybe.
I havent reached the point where you do the Liberation missions (im in sequence 6) I guess its not too big of a deal as I have 5 stores I can trade with, but i tend to have OCD about video game things like this lol
Nah, not necessary at all. You make enough money to buy everything from just normally playing the game. Even if you want to get 100% on the game, you can manually trade a few skins to get the last little bit. The money I have on my character after getting 100% is almost exactly equal to the amount I gained from trading.