Transistors are like 800mA, and gate drivers around 1A (I can't find a store in Au selling 6A gate drivers), in fact - this is the only gate driver I can find:
http://www.jaycar.com.au/productView.asp?ID=ZK8878But it's 1A, I have 800mA transistors already!
You can get TC4420 gate drivers from
http://au.element14.com/, although they probably have a minimum order value for non-trade sales (it's GBP30 in UK).
They look good for the job, however they need 10V gate drive, so you definitely need to use some sort of driver with them. The 3-transistor driver schematic I provided should be OK if you are using a low PWM frequency. It may even be OK at the default Arduino PWM frequency of 490Hz, however I suggest you start with a low PWM frequency (say 10Hz), and if the mosfets still run cool with the PWM at 90% then you can try a higher frequency. Ideally you would use an oscilloscope to look at the gate and drain voltages, then you can get a better idea of the energy dissipation when the mosfet turns on and off.
Don't forget to include a flyback diode across the motor. Its peak current rating needs to be at least as high as the motor stall current, so I suggest a 25A Schottky diode or similar (also available from element14).
