Sites like PSX Place (dedicated to PlayStation emulation) or Gazelle Games private trackers offer curated packs. These require registration and maintaining a ratio (uploading as much as you download).
.cue or .chd file. If using .bin/.cue, keep both files together.scph1001.bin, scph5500.bin, etc.). You must dump this from a real PS1 console. Downloading BIOS is also legally questionable.