If you already have a collection of ISOs, you don't need to go hunting for new ROMs. You can convert them yourself using a tool called (which is part of the MAME distribution). The Quick Steps: Download the chdman.exe utility. Place it in the folder with your PS2 ISOs.
Downloading BIOS files (required for PCSX2) is similarly restricted: you must dump your own PS2 console’s BIOS. The PCSX2 team explicitly advises against facilitating or engaging in piracy.
Sony’s PBP format (used for PS1 games on PSP) also supports compression. However:
chdman createdvd -i "game.iso" -o "game.chd" -c 7