Where To Find Inmate Locator Websites
Most states have online searches for inmates through their Department of Corrections websites. Here is the complete list (also includes county jail rosters) - Inmate Locator.
For California, the Department of Corrections has a 24-hour inmate locator telephone number of (916) 445-6713. You must have either the inmate's CDC number, or the inmate's full name and date of birth to receive information. The Inmate Locator will provide an inmate's location, mailing addresses and relevant phone numbers. They will not provide any future release date information.
Vinelink is another database that provides inmate information at the state and county level for most states.
If you are trying to locate an inmate at the Federal level, try the Federal Bureau of Prisons Inmate Locator page.
If the county or state department of corrections you are looking for does not have an inmate locator on the web, I suggest you call the prison directly. They will tell you if the inmate you are looking for is incarcerated in their facilty.