Are you talking about the small plastic ones that go to the smog valves? I'd probably find a small nail and slather it with slicone sealant and shove it in the hole. The other option is to get vacuum caps at the parts store, but these ones are quite small, I'm not sure they'd have that size. If you do find one thats close, put a dab of silicon sealer in the cap for extra sealage.