I have a 15 gallon 12v pump and I am thinking that the reason they dont put them in anything larger than 25 gallon is durability. I dont think they will last long in constant use. I think they are made mostly for spot spraying. If I were needing a large capacity application, I would look for one of the 275 gallon tote tanks that you can get for free from some manufacturers who use the product in them and cant dispose of them. I got one that was washed clean simply for pickup (they even loaded it for me) Anyway, the tank can be free just for calling around and then add either a gasoline chemical pump or a PTO driven one if you have a tractor. If on a tractor, you could make a simple 3 PH frame and mount it. You should be able to get all you need at TSC, Farm Co-op, etc.
If you must have a 12v pump then buy a couple of spare pumps when you purchase it.