Have you tried Metal Samples.  They can be found at http://www.alspi.com/ms.htm

You did not say what size you needed for your plates, but they may have something that you could use.

Can you be more precise in asking your question as I am slightly confused. Do you mean pure Mg plates or AZ series alloys, i.e. AZ31 and AZ61. Secondly, if you are using the plates for experimental purposes, you need to be sure of the composition, hence you cannot rely on local suppliers. Maybe try Magnesium Elecktron Ltd. for that matter. 

You can get magnesium plates of relatively small size by using stir casting method   process you have to manufacture for your requirement.