How do you get weapons in Roblox?

If you want weapons in a certain non-weapon game, you're out of luck. However, if you mean when you're building, that's a lot easier. The best weapons, but also the hardest to find working, are on the "Free Models" database. You can get to this database by clicking the "Insert" button on the "build" or "edit" modes, then scroll down to the "Free Models" menu, and search for the weapon you want.