To avoid what I just went through, check to see if you already have the cleaning sheets. They apparently came with my printer and are located in an obscure drawer. Pull out the bottom paper tray, turn the tray so the right side is facing you. Look for a 5" wide, shallow tray near the front. I pulled it out while on the phone with a technician and found 5 sheets. I used 2 in the Sheet Cleaning mode (in the printer utility screen). FYI: The window prompt instructions are misleading about paper position. Feed it LANDSCAPE (or wide) position with the notch in the top right corner, adhesive facing to the back. See drawings on that obscure drawer.
Summary of my experience: I called Canon yesterday after the Sheet Cleaning alert automatically appeared after a preset number of prints were made. After the alert comes, you have to hit the RESUME button before every print job until the Sheet Cleaning is performed. Enough of a pain to get this taken care of quickly.
Well, since I couldn't find any part number or detailed info on this in any online manual or forum, I emailed Canon through their website. Their response: "The cleaning sheets are available for purchase through our Jamesburg Parts Department. You may call them at (866) 481-2569 between 9 am and 7 pm ET, Monday through Friday (excluding holidays). "
I placed the order for 2 sheets at $2 each plus $6 shipping and received it next day. It was literally 2 flat sheets in a 24" x 16" x 12" box, 99.99% airbags! Anyways, It would've been great if someone at Canon could've told me that I might already have the sheets. Now I'm working on getting a full refund on my $11---it's on principle.