Interested in not just cutting out with your machine, but writing too?  Here are the pen tools that I know of that you can choose from!  They come in different materials and different sizes, so the price varies a little.  You will probably be able to use more pens than the ones mentioned.  They are useful in gauging sizes that the pen tool can take.  Shipping will vary greatly if you are not in the contiguous states of U.S.

Silhouette on Amazon:

$9.99 + shipping based on your location (free shipping for order over $35) + taxes based on your state – Returnable

All plastic with two screws to be able to center pen.

For ballpoint pens.

Amy Chomas:

$22.00 + $5.00 shipping (flat rate and free shipping over $100) – No Returns

For gel pens like Sakura Gelly Rolls.

Available in a larger size for markers too!  “The adjustable pen holder holds pens up to 3/8 inch while the adjustable marker holder goes up to 1/2 inch.”

HotPaws:

$16.00 + $4.95 shipping – Returnable

Comes with accessories – extra screw and foam pieces to help center the pen.

For use with Stampin’ Up© Pens, Other Pens include Uniball Signo™, Close To My Heart™, Zig™ Pens and Sharpie® Ultra Fine Tip Pens (Only Ultra Fine Tip). Many other pens will work that are smaller including Pentel Slicci, Sakura Gelly Roll Pens, Glue Pens, Papermate Erasermate Etc.

Has an additional version for Bic Mark It pens.

The Sensory Emporium:

$20.00 + $2.50 shipping (+ $2 for additional items bought) – Returnable in 14 days of purchase date

Comes with a popsicle stick for spacing the gap that needs to be there between the pen and the material you’re writing on.

For Gelly Roll Pens, Sharpie Ultra Fine Tip Markers and BIC Markers