First you'll need to track down some suitable plates. Ceramic plates can be good for performance as the risk of breakage adds a little "edge" for the audience and deliberate breakages can add real drama to an act; however for practice something unbreakable is desirable.
Plates for picnics and camping are the best for practising, so try camping and outdoor shops, especially in Spring and Summer. If the plates that you find are too light then stick a few together. This works best if the plates stack together tightly as they can then be glued; if there's a lot of air between them then tape them together at the edge - it doesn't look as good but will work just fine.
You can also get melamine plates which are found in posh picnic sets (try a department store in early summertime) or the restaurant supply shops that seem easiest to find in Chinatowns. These look like the real thing and tend to be reasonably weighty too. Unfortunately they break eventually so get plenty.
Any unbreakable plate that is reasonably weighty will do, get out to your local shops and explore. Of course in performance breaking a ceramic plate can add a lot of dramatic effect; check out charity and second-hand shops for a regular supply cheaply.