when find kittens at shelter?

We went to the humane shelter today to adopt two kittens. We want two so we
can share them better in our family and so they will be happier when alone,
but there were no kittens there. Only older cats...and no bonded pairs
which we would consider too if young.

Is there a good time of year when to expect there would be kittens at the
shelters here in California?

thank you for any input