The shield materia can only be found in the northern cave. You have to enter the northern cave on disc 3 and climb down until the point where you have to choose a direction to go. To get shield, take the left path, then you will have to decide on a direction again i can't remember whether you go up or down but if you choose the right path you should end up in some slimy place which looks a bit like someone's insides. Go to the next room and there you should see a green blob on one of the slimy bits this is the shield materia. Hope I helped =)
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Yes, I'm using codebreaker for FF7 on my PS3 right now and it works just fine.
With a emulator for the PS1
no the game is not for a PS1
The PS1 has 7,918 games bud
Hit the triangle ( for the ps1).