in Fullerton has a better selection than most stores and dealerships in general, but I was not looking at their boots last time I was there.
There is also a cycle gear in Fullerton fairly close to Motorhelmets. They carry a Sedici brand, which I think is a cycle gear owned brand. Sedici boots run a little wider then most boots and that brand seems to be better quality on average than the 'Bilt' stuff.
Irv Seaver BMW has a decent shop.
Extreme supply mentioned above did not have a showroom as of a couple years ago, but the sales guy up front will fetch stuff and let you try it on.
Cycle parts west does not have much street stuff and they seem to have down sized the store recently. Maybe worth a stop anyways since you will be in the area. They usually have good prices on what they have.