Suspension fork top caps
@Michal,  I can't be certain from your picture, but your fork looks a lot like mine.  If it's the same kind of design, then the caps are simply plastic plugs, not threaded, simply friction-fit into the openings.

Here's a picture of the cap area of my fork.



If mine were missing I'd do just what @RoboKaren said, either make a plug out of a genuine cork champagne cork or maybe one of the plastic ones or (probably what I'd do) just duct tape over the area after cleaning the area with soapy water then mineral spirits to remove all surface contaminants and make sure the tape sticks.  You can get black duct tape if you want it to blend in.

BTW I'm assuming my suspension fork uses elastomer "springs" for the shock-absorbing effect, but I'm not positive because I've never been inside.