I build them myself. If you gently take one out or in pieces that you can put together, then make an outline or template and with a jig saw cut your own. Most likely each sill has its own pecularities that make retailing sills prohibitive.
Then too, if the window holds a ac or has a tendency to rot for some reasons you can then change the species of wood to accomodate the situation. Some under AC's work well with 5/4 PT wood.