The cart Andy suggested is basically what my team has used the past few years. They are available from a ton of manufacturers, and are customizable to your own needs. Last year, we mounted a plywood deck for the robot to sit on. If you choose to do this, ensure it is strong enough to handle the robot (If it isn't, injuries will occur).
Also, you should consider your robot's center of gravity in whatever you build (we almost dumped our robot off the side of the cart a few times last year).
Flashy carts are usually a waste of money, I would much rather have a flashy robot
.