Cheap futon mattresses can be purchased at a number of online retailers. The idea of cheap varies from person to person, depending on individual budgets, but discounted and low-cost futons can be found at US-Mattress, Walmart, Overstock, Kmart and Target.
US-Mattress has a page listing a selection of cheap futon mattresses. As of 2014, its cheap futon mattress sections consist of mattresses under $100 and mattresses costing between $100 and $200.
Walmart lists a large variety of inexpensive futon mattress brands ranging in price from about $90 up to approximately $350. Many of the futons are available to order online or for store pickup, but some can only be ordered via a third-party retail website.
Overstock has a selection of low-priced futons starting at around $90.
A search on Kmart for "cheap futon sofa beds" yielded 249 results ranging in price from $99 for a plain armless futon to a little over $500 for a richly-colored leather futon with under-seat storage.
Target also sells futon mattresses and sofa beds. A plain 6-inch or 8-inch futon mattress can be purchased from its website for about $129. Other futon mattresses that come with a simple metal or wooden base or frame start at about $150.