The 120A main breakers that FIRST supplied us with can usually be found at Marine Surplus Stores.
Blue Sea Systems makes something very very much like what we used, if they didn't supply them exactly. The model 7113 seems to be their 120A breaker, but they're available in ratings from 25A to 150A, in that same style.
http://www.bluesea.com/Products/CB/CB.htm
http://www.bluesea.com/catalog_.pdf/...ussmann_cb.pdf
I'm willing to bet that you could walk into almost any automotive or marine supply store, and ask if they have any high-amperage circuit breakers, and they'd be more than willing to help you out. I'm not too sure about pricing. You might have to pay $50 for a good one, but that's just a guess.