I was wondering if anyone can tell me what cities mainly does clients use TER to search for providers. I know it differs in many states.

You can easily see which ones are more popular. Another way is to browse new reviews and see where the areas where most reviews come from. Neither of these methods is fool proof but can give you a general idea.

Anyway, I would say the following cities (or in some cases, states) are the most popular TER areas. (In no particular order, and not meant to be a complete list.)
New York
Los Angeles
Las Vegas
Washington DC
Atlanta
Chicago
Boston
Minnesota
Ohio
Florida
Carolinas
San Diego