I assume you're talking about the black knobs for the sliding bed extender rails? If so, that IS a good question.... Honestly, I would try either a professional fastener store or possibly an RV dealer. You may be able to find one at a lawnmower parts place. Many of the newer mowers with collapsible handles have these same kinds of knobs that tighten after you "unfold" the handles. My Craftsman mower had them. Just trial and error until you find something that fits.