I got mine by writing to my high school ~ 15 years after passing my exams.
They still had my certificates in their files, and told me they never destroy them. This may well have changed now, as when my daughter went to pick up her GCSE certificates she was told that they were only kept for 3 months before being destroyed.
The education authority where you went to school would probably help in this case, but you would have to pay for new certificates to be printed.