Is there a system that fires torpedoes that is available somewhere? Most enemy ship seem to have them.

depends on how much of a distinction you put between torpedoes and missiles

antimatter probes would prolly be the best fit though its classed as a missile in game

otherwise i figure torpedoes require a specially designed ship to use them none of which are available to the player

The large, slow moving missiles that show up as 'heavy torpedoes' when you target them with turrets are not something I've seen available, but I was wondering if there's a militia base in one of the systems that might carry them.

I don't think so, but was hoping maybe someone knew something that isn't officially documented. :)

Maybe a alien tech drop? I know you can get some missile weapons that way only.

I have seen elsewhere, in dev comments way back when the game was "new", that by the time the game was released, the terms "torpedo" (as mentioned in the achievements) and the concept of "heavy missile" were functionally the same.

You can actually see this while keeping track trying of the missile kills necessary to get the Torpedo Buster achievement. if some little swarm missiles fly through your very active broadside, and your see some of them hit and early detonated, then right after that battle you check your stats - no uptick in missiles intercepted. if later the same thing happens with some Commandship firing clusters of straight flying, bigger missiles - check the stats, counted for intercepts. Then you do the same thing upon encountering a Torpedo Barge, etc,, firing those bright spark-like torpedoes - any kills of incoming, by non-flak weapons, also gets counted in the missiles intercepted stats.

BTW. Way easier to get the missile kills for Torpedo Buster achievement by installing a fast reloading broadside weapon, maybe with a Broadside Optimiser, than by trying to use the missile sharpshooting subsystem, IMHO. Install an appropriate "curtain of fire" broadside (and a good shield, and deflection absorber), choose some missile rich missions and then just point-and-time the broadside blasts to catch the incoming clusters of missiles.