As ethel says your school should have all this information. They should have everything on a compterised system and be able to download the information for you. If you ask them nicely they may even type you up a letter on their headed paper listing your results! Obviously this isn't a certificate but they will be able to tell you which exam board to contact - be warned it does cost to get replacement certificates so it might be worth asking whoever wants to see your certificates if they would accept a letter from your school.