Approaches (aside from Erie and stocked or fed fish):
1. Big Rivers. Allegheny Tailwaters. Delaware. Lehigh. You could add the Yough, Clarion, etc. in there.
2. Limestoners. Generally, there has to be some browns but the real trophies are usually where there's not a real strong population of them. Perhaps the stream has a strong wild brown population upstream, but downstream it begins to warm and becomes marginal. That marginal area is often where there's not many fish, but there are a few trophies.
3. They exist in smallish freestoners too. Can be like the limestoners, in marginal areas. Also occurs in streams that are primarily brookie streams. That super deep pool with the big rock undercut that you strangely never catch a brookie out of? Yep.
The constant seems to be that unless it's a huge waterway, trophy browns are NOT found in the same places where A LOT of browns are found. You are not after numbers. You are after a place where lots of food is found, but food is not the population limiting factor. Maybe holding water. Maybe water temp but there's a small spring where 1 or 2 fish can oversummer. Whatever it is, you have a situation where only a few fish can survive, but those that do have an abundant food chain virtually to themselves without competition.
The exceptions to the above are when it may not be a huge, rich waterway, but it's close to one. We said about Erie, the D, Allegheny, etc. Well, the TRIBS to those same waters can have em. Same goes for reservoirs.
In all cases, fish streamers at night or in high muddy water to structure. In smaller water you can often locate them in the daytime and then go back and target them at night.
On smaller waters, approach it like a bowhunter chasing a trophy buck. You're searching for a tiny subset of the population. Once found, you're patterning a specific animal, and then you set up an ambush.