Demo days are't usually scheduled until the weather is more consistently warm. Most Kayak shops are near the ocean an will let you demo "SOME" of them.
The more expensive ones you'll have to wait for an outdoor fair located in or near a lake/marina. Contact the manufacturer to see where/when their next demo fair around the L.A. Area is (you'll have to call around).
In Marina Del Rey, try Action sports @ 4144 Lincoln blvd. Then there's Taylor Kayaks near the Venice Costco on Venice Blvd..
Afraid that's all I got.
Hope it helps.
FFY