I've never used a G12 fixture, but I have replaced ballasts several times in various lighting fixtures.
If you can't get the original style of ballast from the manufacturer, you can often find a similar size one from some place else. You will need to match the ballast to the bulb. You may also need to drill a hole or two so you can mount it.
It's also possible that you can use a different ballast, but due to it's size you'll need to mount it externally from the fixture. This isn't as bad as it sounds.
I do see where Aquatic Life still sells the single bulb 36" fixture. You could get that, and use it to repair your fixture.
All and all, you shouldn't have too hard a time of it.
As for why the light is discontinued I can't speak for AquaticLife. I suspect what happens today is that a manufacturer makes a run of product, and when that sells out they might do another. If it doesn't sell out too fast, they don't make another run. In this case you have a MH bulb that seems to be only 150w, which is a little small by today's standards, and you also have LED lighting coming into wide use.