Looking for slides to make a pull-out bed

I'm designing a bed that fits into an alcove in the open attic space behind the bedroom wall. It would be great if I could mount the bed platform on a large pair of slides so the whole mattress could be pulled out to ease changing sheets, etc. In a sense it would be like a very large cabinet slide out drawer. Does anyone have any idea if these large slides exist. They'd have to be at least 36" long. Thanks.