I think that many LA's are not familiar with what is available in the market because they have never had to source the plants! On large commercial or institutional projects, the LA may not have any local knowledge of the area where the project is being built and may very well be using a "book of lists" reference to specify the plants.

I used to be the ONE. I used to think that every plant I see on the Sunset Western Garden book is available right at my fingertips. I learned over the years it is not productive to select every new/ exotic plants I see on the books & magazines, because the amount of calls & emails I would receive from contractors asking if they can substitute plants during the bidding process. This can get old really quickly, especially when you are working on a tight deadline.

I usually select my plants from San Marcos Growers Nursery, in Santa Barbara, On-line Catalog. Their website is very informative & useful and let me know what is available or not. However, this certainly does not let me know “How many?”.
One day, a landscape contractor recommended me to subscribe email newsletters from local nurseries that are issuing “Available Plant List”. Brilliant! Currently, I subscribe couple email newsletters from local nurseries in Los Angeles. This certainly does not eliminate the calls, but it helped me tremendously.

I'm a landscape designer, and yes, I've found the landscape architects have very limited plant knowledge. I find that they are used to create public spaces or developments. I was working under a licensed landscape architect who signed off on my work, like an engineer can have his work signed off by a certified professional engineer. I found the roomful of landscape architects came to me for plant suggestions because I am all about the plants. Yes, on development projects I am constricted by the city/county codes, but not on residential, unless it has deed restrictions.

To truly know and be able to use all of the available plants suitable for a certain zone, I needed to develop a proprietary, searchable database. When I am stuck by a set of site constraints, ie, shady/swampy/hot, I just search my database. It allows me to create one-of-a-kind landscapes that I know will be successful over the long-run. And my customers are always delighted to have a landscape that is unique in the neighborhood.

Oh, and yes, knowing what is "commercially grown" is another heap of information you need to have at your fingertips. Years of inputting data and now I know what's out there. And I am always pleasantly surprised when I see a nursery offering something beyond the ordinary.

Having previously worked four landscape architects for over 7 years I can tell you that most of what was designed back then was what I refer to as cookie cutter. We took one plant palette and used it over and over... It was simple and easy.