I purchased these fat markers at the Dollar Tree (brand Jot, distributed by Greenbrier International, Inc, which is the store brand). These fit perfectly in the Silhouette!
The smaller chubby markers here (in the middle, the yellow and green that are together) were "Spiderman" markers from the Dollar Spot at Target. They were just a smidge too thin to fit well into the holder, I had to place a strip of masking tape at the top of the marker by the cap so that the holder could grip it. And then a double thick piece of masking tape to act as a stop. Although these were a little more work, the colors are more vibrant, and there were 10 colors for $1.
If you do purchase the Dollar Tree markers, the tape goes exactly 15/32" from the part of the marker that widens out from the tip. Tape one, test it, and if it works, use it as a guide to tape off the others. Just lay them next to each other and tape using the first one as a guide. Have fun!
Oh yeah, I did not get around to publishing the Two for Tuesday deal at LD yesterday - it is the Cutie Patootie Boy Bundle. A great price on a cute set (image is linked).