Well since you finished the game I am assuming you have done most of the homestead missions. If you have never been in a store which I highly doubt, its the sign on the map. Like the euro sign. If you want weapons that you cannot buy in the stores you will have to craft them.
1) You need to do homestead missions to acquire to homested settlers.
2) Then go to any store and go to the edge of the counter and press B in front of the book.
3) Go over to crafting and pick what recipe you would like.
4) It will tell you what you need in able to craft an item.
*If an item is red, that means you do not have it, or not enough of the item. Either do more missions to craft higher quality goods, or craft the item you need.

I hoped this helped, if any other questions just let me know, and I will do my best to help.