You could (besides the recommendations in the thread above) try e-bay.
I often wondered if there is much difference between a clear Pyrex glass plate and one of the microwave ones. I know it will not sit as snugly onto the rack of the microwave but if it can be used as a normal plate then there cannot be that much difference. Kildrought also mentions in the thread above that they have used a standard dinner plate for some time with no problems.