The most legal and stable source. Search for "PS2 CSO Collection" or "PS2 Highly Compressed." Filter by Date Archived (Last 30 days) to find "UPD" packs. Many users upload complete libraries in CHD format (the new gold standard).
PCSX2 natively supports .gz files. The emulator builds an index for these, so there is no performance lag during gameplay. i ps2 highly compressed games iso upd