I am searching for a big chess games database, in PGN or CBH or SCID format. ICOfY Base is no longer available, I think. I found nothing in torrent sites (I am searching for something that is legal).
I use the games published by The Week In Chess, which are published in a weekly basis. These games make a superb database, although maybe you'll miss some historical games. It takes some effort to build the database with all games published to the date, but from then on it's easy to keep it up to date.
It is also possible to download the Million Database and then to keep it up to date with TWIC games.
I know of at least two sources you could use (one is free, the other costs).
- lichess added a "opening tree" with many games categoriesed by the moves you play (you get a list and games that share the same move list). I am not sure if you can get the info easily out of this tool, though.
- chessclub.com or "ICC" offers their database on their website for download for money. Not sure how costly it is, though.
Possibly chess.com. There are millions of games played there, and each one is available for PGN download - for free.