Ive never seen any cheat or trainer to make the collectibles show up on the GTA map.
The only reason you'll want all the Oysters is to increase sex appeal or your trying to achieve 100%. If you just want to increase sex appeal, then there's a trainer that will max out all your levels i.e. maxed sex appeal, luck, weapons, etc..
If your just looking to max out your levels, then a trainer is the way to go. If you have XBMC on your xbox, then use the built in trainer and grab a trainer from maxconsole.net. Here's a link to a guide that shows how to use trainers with XBMC --> XBMC Trainer Guide
The only reason I went for Oysters, Horseshoes, Tags, and Snapshots was so I could achieve 100%. I used maps to find them all and certain vehicles, for instance:
Oysters - Used a boat to go to each locale.
Horseshoes, Tags, and Snapshots - used the Jet Pack to get to each locale.
Here's links to the maps I used courtesy of gta-sanandreas.com
I saved each map picture file to my PC and opened it with a photo editor so I could mark each location I collected.
If you follow the maps by marking off each location, be absolutely certain you mark the correct point you collected from on the map. If you incorrectly mark a location, then at the end you find your still missing an item, then your screwed and will most likely have to check every single location again. This happened to me once and drove me nuts!