I've done just this in a few rooms! The cheapest product to use for windows is blackout cloth for curtains which you should be able to buy from just about any good cloth shop. They can also sell you white velcro - white will help it blend in with your widow frame though it will yellow with time and need replacing. Curtain blackout cloth works just fine with the velcro. You tend to find that if a product is targeted at the photographic market the price increases by a factor of 3-4.