Not sure if this is the right section, but I saw an add for 3 sheets of 6x4 glass, most likely tempered, while browsing St. Louis craigslist. Worth checking into at least if you are in the area and planning on doing a plywood tank (nothing cheaper than free).
http://stlouis.craigslist.org/zip/2961918379.html
http://stlouis.craigslist.org/zip/2961918379.html