The type of door you are looking for is called an accordion door. They are definetly inexpensive and are at Menards, Home Depot, etc. I used to work at Menards in Millwork so I know they are there for sure. Its less than a hundred dollars and can come in a variety of colors. They have some standard colors and sizes but some odd sizes you may have to order for a bit more money. But, the sizes they have in stock are usually enough to cover the opening that you are looking to close off.