Harpoon is available for free online, somewhere, legally, along with the theater packs. Err - just checked. It WAS available online for free but is now part of a bundle pack at Matrix Games. I think I got it from Advanced Gaming Systems but they no longer carry it. Given that Matrix has this bundle for $60, I'd be surprised if it made an appearance here.
This game sucked up sooooo much of my time. Just loved it way back when. I spent hours just looking through the unit info pages, let alone actually playing the game. I dl'ed it a while back and installed it; not sure it aged well. For sure some of the mechanics are sorely dated, but that may be a necessity of the manner in which the game is intended to be played.