What this directory is
High Cheese tracks 335 adult baseball leagues and 2,558 teams across 48 states. Every one is a league somebody is running right now, not a list bought from a data broker. Where a league has gone quiet and its website is a parked domain, that gets said plainly instead of dressed up. A directory that pretends every entry is thriving is a directory nobody can use.
How to find one that will take you
Start with your state, then look at the age division. Adult baseball is organized by decade more than by skill: an 18+ league and a 45+ league in the same town are different sports played under the same rules. Most leagues run one division and a few run six. If the one nearest you only fields 25+, the next town over is usually worth the drive.
After that it comes down to the calendar. Sunday-only leagues are the norm because everybody has a job, and a league that plays weeknights is telling you something about how many of its players have small children. Read the schedule before you read the roster.
If you are not on a team yet
You do not need one. Put yourself in the free agent pool and managers looking for a body at your position will find you. Rosters turn over every spring, and the manager who needs a second baseman in March is not going to find you by accident.
If your league is missing from this directory, add it. The record of a season outlives whoever ran it, which is the whole reason this exists.