Marriage Licenses are obtained at any County Clerk's Office. It does not have to be the county where you reside or the county where you plan to be married. The only requirement is that once you are married you must return your signed marriage license to the county that you obtained it from. I have worked with Los Angeles, San Bernardino and Riverside County Clerk's Offices. Riverside is the easiest and fastest county to deal with.
I strongly recommend obtaining your license from Riverside. The office on Gateway Drive doesn't get a lot of traffic. If you avoid lunchtime you can be in and out pretty quickly without having to stand in long lines. The other nice thing about Riverside is that if your license is submitted for recording by Tuesday of any given week, your recorded document will be available for purchase Thursday of that same week. Other counties take from 2-6 weeks.
The Riverside County Clerk's Office is located at:
2724 Gateway Drive
Riverside, CA 92502
8:00 a.m. - 4:00 p.m. / M - F Closed for Holidays.