Sources for scenarios for Arkham Horror
Custom Scenarios were used in 2 Arkham Horror Leagues, organized by Fantasy Flight.

You can find the whole scenarios from the Arkham Horror Wiki Scenarios Category page, along with pdf printouts of each story. There doesn't appear to be a summary of season 1, however.

Compilations of the league scenarios nicely assembled were contributed to the file section on boardgamegeek in pdf format:

League 1

League 2