I found some inexpensive markers that fit (a bit tightly) the pen holder in the Cricut Expression.
They are $2.97 at Walmart, and can be found in the kid’s coloring section with the crayons. They are Cra-Z-Art Washable Mini Markers. I didn’t notice the “Washable” bit until just now as I was writing this post. The mini Sharpies might work too, but they are a bit expensive. I’ll probably browse the shelves at Office Depot to see if I can find the Sharpie minis.