Yes, I know it's stupid, but if you play for long enough, you'll get randomly dropped weapons. These may not suit you, so see step 2 for more on trading n ♥♥♥♥.

You can do this through Steam or through the game. By standard methods, that is. Using a website- scrap.tf - makes it easy to get what you want by trading your random drops for specific weapons. Keep in mind that you will need to have your mobile authenticator activated for at least a week for this to work.