Yeah.There are some small stores selling those kits. They should have the tiny screw and tiny screwdriver. You can replace it with the tools by yourself. Or if you don't find these stores, just go to find a optical store, they will fix it for you.
Maybe you can have them fixed. I suggest you to visit the optician who you bought your sunglasses from. They know your sunglasses better than other people. And there might be some extra screws in their shop. If they have, they can give you a new screw and fix your sunglasses.
It is a little difficult to find a screw jsut the same as what you have on your sunglasses. But you can find some screws with other color or shape which can replace the losing one. Have a try! Good luck!