If you cannot, or do not, get permission to use a work, you must reevaluate your use. You will either need to use another resource or re-examine fair use and see if you are able to change the way you want to use the work so that the use will constitute fair use.
You can ask a copyright holder for permission to use a protected work if none of the exceptions to copyright apply, and there is no license permitting you to use the work.
Be sure to explain how you want to use the work and to ask for specific permission to use the work the way you want to use it. Just as with licenses (which are actually a form of permission), your use must be consistent with what the copyright holder has given you permission to do.
Who/How to Ask
Permission must come from the holder of the copyright -- which may or may not be the creator of the work (because copyrights can be transferred). If a work is published, you can ask the publisher whether they hold the copyright (or know who does); for unpublished works you can ask the creator.
Ideally, you should get the permission in writing. However, written permission is not required. If you receive verbal permission to use a work, record the details and send the copyright holder a written communication to confirm the permission. Make sure to keep the record of permission being granted in case there is an issue over your use in the future.
You may not be able to identify (or find) the rights holder. Due to the increasing length of time that copyright protects a work, there is a growing problem with what are called "orphan works." These are works which are still protected by copyright -- but whose owner is unknown. [Copyright still protects such works, and you cannot use them unless one of the exceptions applies.]