typically, your mechanic will have left them off after doing a service
pisses me off when they do that, it saves them time, then when the customer doesn't notice so they throw them out, or if the customer does notice act all innocent, "oops, forgot!"
their purpose is to protect engine areas from salt spray, and provide some wind noise reduction in the cabin and some aerodynamic benefit, so most customers won't notice for a long time, and then won't realize their mechanic left them off on purpose
grumble
Oh, and above advice was spot on for the fasteners: The threads are M6-1mm. Their heads are either 10mm or 12mm (socket size), use stainless steel for longevity, and add a washer for load distribution.