I am not sure what you mean by "strongest" with respect to diamagnetic materials - I am assuming you mean just "how strongly diamagnetic" they are? Getting plates of the size you mention, let alone at a cheap price, may well turn out to be a big challenge. I don't know what your planned setup is, but my sense is that your experiment will also require those plates to be very flat for reliable results? That alone would be hard to achieve for a material such as lead or even copper (to get two large plates to be the exact same distance throughout their surface). Maybe you can do some calculations to see what the effect of unevenness of the plates or a variation in their spacing over their area would be, or try and come up with a setup that requires smaller pieces of the materials. The smaller, the better, most likely. To get a hold of the kind of plates you are thinking about, probably more by borrowing then buying, you might want to try institutions that give lectures or demonstrations of basic physics - whoever runs the beginning classes at UCSB or City college, or science museums. To buy them, maybe a supplier of science equipment for schools and universities is a starting point; maybe you could check with your science teacher or look online? You could call a place like that and see if they can point you in a useful direction. Hope that gives you at least some leads to pursue...