Electric Motors for a robot

Hey, I have an Arduino card an I would like to start making a robot - yep, really original for Arduino - and I need, of course, electric motors, but I don’t know which one I should use for my project and how many I need (there will be 4 wheels but they probably don’t need to be each motorized). I just wanna know on which website we can find cheap motors. I can do it on my own, but if you want, here’s the inforamtions : -Weight: I don’t really know but less than 2Kg -Shape: It will be a rectangular base (10x5x16)cm and ontop there will be different things such as a cell phone support and the box for the batteries (the number will depend on the required voltage / amperage of the motors / arduino card)

Thank you, and free feel to ask me more precision !