The standard sizes for sheet pans in the industry (at least for the US) are based on the "sheet pan", and are approximately as follows, at the edge of the rim:
- Full sheet: 18 x 26 inches
- Half sheet: 18 x 13 inches (I suspect your jelly roll pan is actually this standard size)
- Quarter sheet: 9 x 13 inches
Your cookie sheet is not one of these standard sizes, so you are unlikely to find a common silicone mat (or pre-cut parchment supply) sized for it. Of course, you can always use the next smaller size....
In fact, you are unlikely to find silicone mats for sizes other than these, and cutting them is definitely not recommended. Mats sized for standard sheet pans will be just a bit smaller.
I do not note any odd sizes on Amazon, but that doesn't mean specialty sizes don't exist. They would be a niche item, however.
On the other hand, sheet pans are very inexpensive, and incredibly versatile. One of their myriad uses is as a jelly roll pan. You could certainly buy half sheet sized mats and pans, and use them together, for jelly rolls, cookies, roasts, or almost anything else.
Another alternative is to purchase baking parchment, and cut it to size as required. You may get 2 or even 3 uses from each sheet if you are careful.