Is there a website that will let me search for master-level games from a specific chess position and download them into a PGN file for free?
Sure - I imagine there are quite a few. Here's one: https://www.365chess.com/
- Expand Resources, and click on Board Editor
- Clear the board, and set up whatever position you want. Make sure you state which side is to move, and whether castling is available still ...
- Below the board copy the FEN of the position
- Expand Search, and click on Game position
- Choose whether to search in the Openings Explorer or Games Database
- Click on Import FEN, and Paste in the FEN you copied from your position. Click Load
- Click on Search!
I think you have to create an account (it's free) to download PGNs, but many of the sites work similarly.