This is the best map I've found on the Internet: Oysters Map
When I first started playing (PS2) I printed it off. Put it in a Page Protector and used a China Marker (office supply, like a crayon pencil) to mark off the locations as I found them. Then erased the marker to reuse the map the next time I played.
The Oysters usually missed are the one in Glen Park, In the Swimming Pool of the Mulholland Mansion, one at the bow of the Automobile transport ship.
There's not much we can do for you without discribing each and everyone of them.
Tricky ones to get are the one in the Submarine Pen and under the bow of the Marine Attack Transport.