A number of other hatcheries sell them too.
Most of them seem to be sold out at present, but that is true of most breeds at most hatcheries I check during this season.
Some examples:
https://www.idealpoultry.com/product/1419/44
https://www.welphatchery.com/layer-type-chicks/california-white-female/
https://www.privetthatchery.com/product/1043
https://www.cacklehatchery.com/product/california-white/
California Whites are a cross of White Leghorn mother and California Gray father, according to several of the hatcheries I checked. To get more you would have to find both parent breeds and repeat the cross.
Of course White Leghorns are easy to find because they are sold by almost every hatchery. California Grays are not as common, but I find them listed at several of the hatcheries that have California Whites.
You could probably cross your current hens with a rooster of either parent breed and get chicks similar to your California Whites. They would be 3/4 one breed and 1/4 the other, rather than 1/2 of each, but they should still be good layers with small appetites. The colors would be a little different: the 3/4 Leghorn would have some chicks all white, while 3/4 California Gray would have some chicks black with white barring. Either way should give about half of chicks that are white with black spots like the California Whites.
Crossing your California White hens with an Ameraucana rooster should give daughters that lay blue or light green eggs. Compared with California Whites, they would eat more and lay less. But they would probably be better than pure Ameraucanas ("better" meaning they eat less and lay more.)
The crosses would have pea combs (bigger than the true Ameraucana pea combs), and they would have muff/beard on their faces.
The color of the chicks will depend on what color the Ameraucana rooster is. For most colors of rooster, you will get some of the chicks the same color as California Whites (white with black spots), and other chicks are likely to be black with white barring, or plain black. Some roosters will give chicks of other colors than those. Certain colors might be male-only or female-only in such a cross, but I do not think there is any color of Ameraucana rooster that will let you sex ALL chicks by color.