You have lots of options here -- really! Approach this search the same way you would for any other vendor. It's fine to interview more than one officiant before you choose. A major thing to remember is chemistry. You want someone you feel comfortable with and who can help you create your ideal ceremony. Here are the four most common officiants.
Celebrants
A certified celebrant has training and certification from an organization like the Celebrant USA Foundation & Institute. They're usually unaffiliated with any religion and can perform secular and interfaith ceremonies. You can find one at CelebrantUSA.com.
Clergy
A great resource for finding pastors, priests and rabbis who are open to performing interfaith ceremonies is your local college or university. Those studying to become clergy members often serve a diverse community and are used to working with clergy from various faiths. Another option is choosing an officiant who's retired and may be willing to perform a more lenient, flexible ceremony.
Interfaith ministers
The main focus of this type of officiant is to bring together people of different religions. Many interfaith ministers define their work as being outside church walls by working in community service and spiritual counseling. Find an interfaith minister at Seasons-of-Life-Ceremonies.com or InterfaithFamily.com.
Friends or family
Another option for you to consider is to have a family member or friend marry you. It's relatively inexpensive and easy to get ordained online. Try Ordination.com or Monastery.org for more information. A major caveat: This isn't allowed in all states.