I think you can just call the Disney Activities number, don't know it offhand but I'm sure it's on the website, and you can reserve one. I think you can reserve them at 180 days out and I'm pretty sure you don't need to be staying at Disney but you'll want to call as soon as the lines open.
Just keep in mind if you're talking about the Grand I, I'm pretty sure there is only 1, (two at most). They do have a number of smaller pontoon boats that can also be reserved for fireworks cruises. If you don't want it for the fireworks, I'm sure it's much easier to get.