No idea where to buy them. They would be easy to sew some up if the windows are small. And if the look does not need to be pretty you can get panels of foam insulation from a store like home depot or lowes and all you have to do is cut it with a knife to fit. You can just set them in the window frames and remove when you choose to have light. But they are not decorative. they are not expensive though and do insulate well.

Take a look at the insulated weaver's cloth curtains at Country Curtains.