The last of these, that we have record of, seems to have occurred in 1880, in San Diego County. This is hinted at by a couple of modern publications. The 1985 book A Spaceship Called Earth: Our Living Environment mentions it, and though we only have the snippit view, it does hint at a primary source (emphasis mine):
The last one was at Pala, San Diego County, in 1880.
In the Haynes' book, a California pioneer, Major Horace Bell,
described that supposedly last bear-bull fight at Pala. it pitted a
grizzly captured on Palomar Mountain against four bulls in an
adobe-walled arena. Bells' account went something like this...
If you want all the details, the West of West blog site has an article Grizzly Battle at Pala with the full story. The article does discuss the original source of the story, which was apparently published in Horace Bells' book On the Old West Coast: Being Further Reminiscences of a Ranger
There is of course no way to guarantee this was the last of these entertainments, but it seems to be the last documented. The state of California passed laws concerning animal fighting in 1905 (it was a misdemeanor):
8 Any person who shall cause any bull bear cock dog or other animal to
fight for his amusement or for gain worry or injure each other or any
person who shall permit the same to be done on any premises under his
charge or control and any person who shall aid abet or be present at
such fighting and worrying of such animal as a spectator shall upon
conviction be deemed guilty of a misdemeanor
The last California Grizzly Bear was killed in 1922.