For ordnance, I would say maybe 1-1.5 mm cylindrical strip styrene. Then an assortment of other small rectangular, triangular, anything you want really. Or, use tookpicks like these for torps:
They are what I used.
Similarly for cruisers you'll want a good pick of strip styrene, whatever catches your eyes, size wise or shape wise. You'll also want a couple size of sheet styrene, 2mm and 1mm should be good. 2mm for base structures and then the thinner stuff since it's easier to cut for details.
I just buy it at my local hobby shop, another small package every time I go, whatever catches my eye that day really. It's not really very expensive and even the small packages go a long way in terms of shipbuilding or any scratch building really.
