We used 2 m&m tubes layered with 2 quarters and a penny. It seemed like we did a lot and didn't run out. You can usually find them in the gift shops of most rides. There were a lot of machines with 3 characters each at the Contemporary by the elevators. We also tried to get all of the ones in World Showcase. My girls loved them and always wanted to stop. They were a big hit with them this trip.
Just a note: I carried my m&m containers in my carry on and had to wait for a more thorough bag search at the airport. I guess the quarters need to go thru the xray separately. I didn't know that and it slowed us down a little.