If you do not find a set on Ebay or here, you can easily have them fabricated at a stainless steel fabricator by giving the fabricator the measurements and they will easily be able to create a set for you to place inside the brackets.
I believe part b-241677 is what you're looking for. Can anyone validate? These are the ones I bought for my kds20. These also fit my 21. They should fit the 18.
I bought mine on eBay and just checked there are none right now.
I do not have the panel that you need. What I have is for an older model. Measures 26" x 10". Screw holes on the sides. Actually I'm not sure what it fits.
I picked up these (link below). They cut the panels to your dimensions. They are made of 22 gauge SS so they're a bit thicker than OEM panels. They came well packaged and in three days.
The 19 and later series had framed panels, not the 18 and older. There were some decorator frames to fit panels in some of the older models, not sure if the 18 series still had these available.
Janelle,
I have a few sets of the trim kits with panels in them already which will accommodate the stainless steel panels you want to get.
The postage will be more than what I would be asking for them so let me know if I can be of assistance to you in this project. Stainless panels make any KitchenAid look fantastic and contemporary.