Tilework detail at Hazrat-e Masumeh mosque in Qom, Iran. Photo taken by me.
The Wade Photo Archive has a wonderful website of 4000 pattern images you can browse for design inspiration. Much of the work is copyrighted so you can’t exactly grab and go, but just looking through centuries of incredibly ornate patterns can really get your creative juices flowing. The way the site is organized is great and includes information on origins and significance, as well as drawings, diagrams, and analyses. I love their browse by region feature.
Eye Candy: Islamic Art Pattern Archive