http://www.ebay.com/itm/Antique-porthole-glass-16-diameter-3-4-thick-Nautica... In the style of John Dobson, for a hundred bucks and free shipping, this could be a steal. It has some substantial edge chips, but you could probably have it reduced in diameter locally for not a lot of money, and get a clean 15" blank. There are some local outfits with water-jet capability that would make the job a snap. If the chips are on one side only, you could just ignore them and grind the good side.