Bring along a film cannister or a M&M minis tube and stack your money in it (two quarters, one penny, two quarters, one penny, etc). That way you are ready to roll. We put most of our purchases on our debit card and hence carry very little cash/change. It was a pain in the neck to have to get change at the register every time we found a machine. I highly suggest carrying quarters and pennies around specifically for this purpose! And bring along more than you think you will need! Most machines can make about three different designs. As such we made many more pennies than we originally planned. The books are available in most gift shops throughout WDW. Right near the autograph books usually. (Remember to put your name and address label on it in case you misplace it!).
It was truly a highlight of our first family WDW trip, especially since our kids were afraid of more of the rides than we anticipated. It was like a treasure hunt to try and find these machines..................P