With a sudden leak of that size, I highly doubt that a stop leak product will give you lasting (if any) good results. A/C stop-leak products are more suited to small gradual refrigerant losses, IMO.
Also, you should look carefully at the A/C lines and the A/C compressor area and look for dark smudges/areas where the PAG oil will indicate where your leak is. Pay particular attention to the compressor as blown compressor seals are a likely culprit.
If you're not up to the task or cannot find anything, I suggest a local A/C shop that will do a system analysis (often for $25-35).
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s