Only Law Firm in Town

The law firm at the center of the show. You'd think occasionally, the plaintiff in one of these cases would pick a different firm. Instead it seems like there's only one law firm they can possibly go to.

When you have only one lawyer in town, he's usually barely making a living and is just hanging on. Once you have two law firms in town, we discover that both lawyers are now quite prosperous members of the community.

—Typical lawyer joke

A sister trope to Only Shop in Town.

Examples:

Comic Books

In the Marvel Universe, pretty much every time anyone in New York needs a criminal lawyer for anything, they'll call in Nelson and Murdock, unless the writer feels like being different, in which case they may go for Jennifer Walters.

Live-Action TV

LA Law
Ally McBeal
Picket Fences had all manner of issues show up in cases plead by one particular attorney to one particular, exasperated judge.
The Practice
Boston Legal to the point where they occasionally represented both sides in a case.
Kingdom: Justified in that Market Shipborough is a rather small town; there's probably another law firm in town, but just the one. Or maybe two. But no more.
Eli Stone