The easiest time to find any particular plant in stock at local garden centers is during their normal bloom time.

So call around to the larger local garden centers and ask "Do you sell X during the growing season?'

If they say "yes," ask when that will be.
If they say "no," ask if they know someone who does.

This IS the blooming season (as well as the best time to plant). The excuse I get is that where the commercial nurseries are, the nights are too cold.

If you know what you want, custom-order it. It will cost you an arm and leg (don't ask how I know that), and you may have to wait a while before it comes in. But I wouldn't have gotten my glorious and unusual variegated liquidamber tree if I hadn't made my gardener go order one for me.

I live in Oakland, you live in Berkeley: there are only two sizable nurseries. East Bay and Berkeley Hort. They both use the same growers as everybody else, they just get the best quality and are who the pros use. Call them but wait until after the holidays; nobody buys landscaping right now so usually the businesses take a break until after New Year's Day.

OP said "This IS the blooming season..."

As I recalled, peak bloom in my garden was considerably earlier in the season.
Typically, flowering plants hit the sales racks at the very beginning of bloom season, then supplies trickle out as bloom time winds down.

Jean,
who gardened in Long Beach, CA, for 30-some years.