Use your web browser's Find function (in Firefox, it's under Edit) to look for "50m" for 50 mile ultras or ", AZ" to find ultras in Arizona. Other sources include: UltraRunning magazine; UltraSignup and Trail Runner magazine.
Listings in BOLD are advertised in UltraRunning or have submitted updates to me (thanks!)
"http://www.Run100s.com/races.htm" (updated: 12/17/24) StanJ@Run100s.com