If you're looking for decks produced by USPC, they're generally available in some limited variety at places like War-Mart or BJ's Wholesale. Occasionally, they also have deck design exclusives, like a particular color scheme or a particular design that's only available in their store.
There's a variety of Theory11 decks that can be found in shops like Barnes and Noble, as well as a few specialty decks themed to pop-culture topics like superheroes, Harry Potter, etc., but those themed decks tend to be of lower quality.
If you want the best-possible selection of decks in person, your best shot would be to go to a local magic shop, especially one that's less tourist-oriented and has fewer novelty/gag items and is more focused on selling to magicians. For example, in New York City, two good examples of this would be Tannen's and Fantasma, both located in Midtown Manhattan. They will generally have a large variety of the kind of custom decks you see people talking about here, anything that's currently available.
The absolute best available selection would probably be if you went to a major deck dealer's retail location, and for that, I can think of only one: Gambler's Warehouse, outside of Dallas. They used to be the Gambler's General Store in Las Vegas but have changed hands once or twice over the last handful of years. Most of their stock is available at their location, from what I've been told - decks they produced as well as other companies' decks and the USPC standards.
Best of luck in your deck hunting!