The distortions ("waviness")are caused by inconsistant thickness of the pane. It was inherent in the old "sheet glass" mfg process. Modern "float glass" process has eliminated that characteristic. "blown glass" is yet another item, usually involving more elaborate effects such as swirls, colors, bubbles, etc, and more expensive. Look for "sheet glass" again, its still widely available. Though economically your best resource may be salvage yards.