Basically, a small mount is a small mount whether you find it on an Onslaught or a Lasher. Ammo's included with that. Given the same number of ordnance points it already should be - and four-six OP is about as much as a small mount should be. Low-ammo variants that cost 1-2 OP are already a thing. If you want a bit more endurance, get expanded missile racks - that should give you more than enough breathing room for what missiles are actually meant to do. If you did boost the missile supply of bigger ships as suggested, do note that this would, for example, allow an Enforcer to cart around forty-eight sabots or sixteen reapers. Or twenty-four sabots and eight reapers, for that matter.
With a few exceptions (Swarmers for PD, Javelins, Salamanders, and depending on how you use them Annihilators for support), missiles are simply not meant to be a sustainable weapon (barring perhaps a Gryphon-mounted Squall). There's only one ship in the game that innately focuses on missiles to the detriment of other weapon types, that being the aforementioned Gryphon which was designed in an epoch where naval strategist favoured heavily specialized designs. Basically, weapons in Starsector are modular, and as a consequence it doesn't make a lot of sense for larger ships with the same modules to function differently (unless they are specifically designed to do so, as with the Gryphon or Condor).
Beyond that, more missiles are really not needed, especially since the number of missiles a lot of ships come with are already more than enough to instantly turn the course of a duel if applied correctly. And of course, I imagine most people have been overloaded at least once within the midst of a group of missile-armed AI frigates and promptly turned into dead meat.
Consider the Harpoon - yes, one rack only has three shots, or six, and it's easy to throw them away to no effect, but the whole idea with missiles is that you use them at precisely the right them for the right job to an overwhelming effect. Harpoons, Hammers and Reapers grant tremendous damage potential versus unshielded, overloaded or flanked targets (and the torpedoes can sometimes be used to put the enemy between a rock and a hard place if their flux is already high and they can't block the torpedo without overloading). A Lasher armed with any of these can pose a serious threat to much larger ships as a result. I would strongly disagree with TaLaR - they're not just for AI vs AI only battles. I suspect he misspoke and was more specifically referring to the Tournament's infamous missile-centric-to-the-exclusion-of-all-else strategies, which would indeed fall flat if you found yourself even moderately outnumbered. Otherwise, as long as the numbers are within a 1:3 ratio, you can always have enough missiles for them to fulfill their intended role.
At any rate, small missile mounts work nearly as well for big ships as they do for small ones. The burst they provide is universally useful, especially considering the high rate of fire and lack of flux cost - there are no ships in the game that can simply ignore a trio of sabots or a single reaper. Medium and large missile mounts - if available - provide all the extra desired capability on a ship-by-ship basis. And the Expanded Missile Racks hullmod doubles the ammo capacity of your missile launchers for a very reasonable cost in ordnance points. On a larger ship, that's as close as you can get to sustainable missile use, and it's as close as the game can really allow you to get to it without them dominating the game.