I have one in my kitchen and I had my sister make mine. She bought the foam ( good thick quality and put batting around it) then she made the cover. I found the fabric which is perfect at a fabric store on the discount rack. I really lucked out when I found it.
THe decorator I was using for the window treatments wanted $700 for the cushion and I wasn't even crazy about the fabric she picked. THe cushion insert was around $130 and I think my fabric was around $30!! What a steal...
There are a couple of people on here that sew for a business - maybe they can make them for you.

Etsy is a great source, and you can check the reviews first. From their search box, just type in window seat cushions.

Couple of things to consider: Will the covers be removable? It is a bit more difficult to make them that way, so they will be a bit more expensive. But well worth it in my opinion because of laundering options. But of course you'll also want to check in that case, on whether or not they will pre-shrink the fabric for you.

I have a shop on Etsy where that is my specialty. If you want more information, you will have to email me privately, because I can�t advertise here, of course. I will say, however, that all of my cushion covers have a full length zipper on the back so they can be removed for cleaning, and I do NOT charge extra for this. In fact, I would have to charge more for those that were not removable.

What is important is the quality and density of the foam, and making sure that the foam you use is wrapped in dacron batting for additional durability and softness. The dacron wrap protects the foam from excessive wear caused by people sliding onto or sitting and moving on the cushion, both movements that could cause foam to crumble if not properly protected. The dacron also helps with soil resistance.

And, I will say, my prices are no where close to being as high as what jeannieo1�s decorator wanted to charge for a standard window seat.