Sallygold!
Gardeners Supply is a mail order company located in Vermont and they have these AquaSav™ Windowbox Liners in 3 different sizes; 24, 30, and 36 inches. They're priced in sets of 2.
http://www.gardeners.com/buy/a...
If you're interested in these....
They also have Hayrack Hanging Baskets
http://www.gardeners.com/buy/f...
You could contact them regarding the 18 inch size. When they state, "This Product is No Longer Available", they could be just temporarily out of that size and may have it back in stock soon. They do have the 14 inch size available now.
They offer Hydro-Mats that you could use in the above item (Hayrack Hanging Baskets), if you wished.
They come in: Set of eight 7" x 7" mats or 15' L x 21-3/4" W roll
http://www.gardeners.com/buy/h...
They also offer:
They also have AquaSav™ Replacement Liners, Set of 3
http://www.gardeners.com/buy/a...
They come in 12, 14, 16 and 18 inch sizes.
I've ordered from Gardeners Supply many time for about a decade and have been happy. I just haven't ordered of the above items from them yet. There are a lot of positive reviews for each of the products that I've listed.
Hope this helps you.