This question comes up from time to time and it always seems that no company offers glass in the shape/thickness we knappers need. Well, that's true for the most part but there are companies that sell suitable glass. It just costs a fortune.
Bullseye Glass Co. offers glass "billets" that are 5" x 10" x 3/4" thick in various colors. I didn't see any blue on their website just now but I think you may be able to call them and ask for it. The problem is that these "billets" start at $52.00 each!! (bullseyeglass.com). When you go to their home page, click on the tab that says "Billets".
If I want good quality glass at a very cheap price and just the right size for knapping medium-sized points, I buy some Perrier water in the green glass bottles. The bottoms are thick, relatively flat and can be cut in half with glass cutters.