Maggie Lou, I don't know if you use Facebook, but there are a few sites on there for us Accuquilt folks. Great place for not only buying used dies (use some caution, make sure to ask how often it's been used so you know the blades are still sharp)....but the best part is being able to ask questions from folks with similar machines.
Just a thought
Oh, as to your question, OnebyOne is right. Although I have used bigger mats for small dies in a pinch (i.e. when I'm too lazy to get my butt off my chair to get the correct mat) LOL
Enjoy!