Some of your options are:
- See if the instructor will issue a rank/instructor certificate or a letter of certification on school letterhead; anything official that may be accepted by other organizations.
- Check with the organization that certified the instructor to see if they will issue you a certificate.
- There are many organizations that may be found on the Internet that issue rank certificates using information you send them. Since there are no legal requirements that govern rank certification, the validity of any rank certification is in the eyes of the beholder.
Since there are no laws governing the martial arts, anyone can open a school or club and teach without any type of rank or instructor certification. You can create your own certificates and issue them.
YMCA's, YWCA's, community centers, junior colleges, etc. are good places to start teaching classes and then work up from there.