You may contact the National Security Authority (NSA) at the Finnish Ministry for Foreign Affairs. Depending on the security level required in the duty, you have to fill in the appropriate security clearance application which will then be forwarded to Supo via the NSA.
After the personnel security clearance vetting has been completed and the NSA has received other information it possibly asked for, it may issue you a Personnel Security Clearance (PSC) Certificate.
Often in these situations, the background shall be inspected and an extract from police records is usually enough for doing this. The extract may be ordered directly from the website of the Legal Register Centre.
National Security Authority