2 quick questions, 1. what type of bb gun should i use for a Fletcher class destroyer.? 2. can i buy them some were or do i have to build them myself.?
Buy one from Strike Models or Battler's Connection. They take Paypal and it's easy. To answer #2, you CAN build your own, but that doesn't mean it's a good idea. Buying one means that it'll work and you won't go to battle with a gun that leaks or jams up or spurts. One cannon costs like $35 or so, and it's easily worth that in terms of saving yourself stress and build time.
No. Those you would have to make yourself. However, single-barrel torpedoes can be made using fairly simple tools to slightly modify a bunch of off-the-shelf parts. What ruleset are you building for?