Contact the graduate school or schools that initially received your GMAT scores in 1990. If you attended this school, the administration will be more likely to have your GMAT scores on file. If you did not, it is unlikely the school will still have your scores on file.
Contact the registrar's office. Ask if your GMAT scores would be listed on your transcript. Request a copy of your transcript and test scores from the school you attended. You may need to pay a small fee in order to receive a copy of your transcript.
Ask for assistance from a manager in the registrar's office if your transcript does not include your GMAT scores. The manager may be able to access additional information at the school.