I have used Perma-nest trays for many years, my older trays were much thicker and lasted forever if I did not abuse them by leaving them out in the sun during the summer. The quality is not what it used to be, the last batch of Perma-nest trays I bought are much thinner and more brittle, and I have already cracked a few edges. I don't plan to buy anymore at their present quality and will just double up on the black plastic kind where I combine a mesh tray with a solid one, or use a broken tray inside a new one.
Perma-nest is sold by various garden catalogs, including Park Seeds. This place sells in multiples of six.
http://growerssolution.com/PROD/perma-nest-plant-trays/PermaNest