Do you have an intact backup of the database produced by a pg_dump command or are you just working with the raw database files (such as from a salvaged drive)? I believe the plain-text backup format, which basically just spits out all the SQL commands to recreate the database and load the data, is forward compatible with all versions. Version 11 introduced breaking changes in the archive backup format, so you would have to install version 10 to restore to, which is the oldest version currently maintained.
If you're working with just the raw database files... not sure of the best approach. You can try websites like oldversion.com (if you trust them). Postgres doesn't host compiled versions for Windows except for their Enterprise DB, so you won't find an official Windows installer that old. Safest approach would probably be to build from source, which might be easier on a Linux system. Is the end-goal to restore a legacy application to the same OS or are you just trying to salvage the data?