For permission to use text materials published in the U.S., go to the Copyright Clearance Center Pay Per Use Search. It will ask questions about the material you want to use and how you want to use it, and then give you a Quick Quote if they content can be paid for through the CCC, or give you contact information for the copyright owner so that you can negotiate permission with them directly. Here are the CCC Pay Per Use instructions.
If you can't get permission through the Copyright Clearance Center, you need to contact the copyright holder directly.
Once you've identified the copyright owner, you need to contact them in writing (electronically counts.) If there is no web form, copy and paste this Permission Letter Template for your email and modify it as needed. Make sure to get and keep a written copy of the license - verbal contracts are unenforceable in practice.
You can use certified mail if you are concerned that the copyright owner isn't receiving your messages. Keep a copy of your letter.