The OP is referring to the bed that sits on the couch in the slide. It is not a flat mattress, but instead has an inflatable spacer that keeps part of the mattress off of the floor.
I just called my dealer and he didn't know who made it, but said that the last one he ordered was close to $300!!! You might call the dealer that you bought from, and he may be able to give you some help.
I personally would spent a lot of effort to find that leak for $300. The problem that I have always been afraid of is that it will leak inside of, or around the inflator motor. That might be something that you would have a hard time fixing. My son uses ours, and he likes it.
Edit: just found this, might help. http://www.amazon.com/Endura-EaseTM-...ords=rv+airbed