The dwarf species dont really have the same "im gonna bite your hands off" attitude of the red snakeheads but they are still fairly aggressive and 100% predator.
You wont find a tank above 6x2x2' in stock at any fish store, tanks above that size have to be custom made. There are a number of companies that advertise in Practical Fish Keeping that specialise in custom build tanks, some will even build the tank on site so you dont have to worry about moving it. Most good LFS's will also have a company they use for custom build tanks too. However dont expect it to be cheap, you will be looking at around 5 or 6 hundred pounds to have a 120x30x30" built and then will need to spend around that again for filtration, heaters and lights.