I'd second detonation films. They have loads of big explosions (and lots of other things going bang - rockets, muzzle flashes, etc) They REALLY love blowing stuff up. THe best part is loads of their stuff is FREE!

I've tried to make a convincing big fireball explosion with Particle Illusion but it still looks like a simulation to me (but then, I am probably not very good. I have seen nice stuff done with it.)

Then there's Andrew Kramer's Action Movie Essentials which I seem to remember has some big fireball explosions, and they are all rendered with the alpha channel transparency already.