I'd just replace the box springs and mattress. Sears always has good deals on those items. You have to watch out for some of the low cost bedding places as they just recover old mattresses and box springs and that's besides against the law just plain unsanitary.
As for sleeper sofas and futons the mattress is too thin to be comfortable for every day use. It has to be thin to be able to fold up. I have yet to find one that is comfortable enough to give up my Sealy mattress for. Besides try finding a mattress for a sleeper or futon for a resonable cost.