AR15 Plus Store - custom firearm parts, kits, AR15, AK47 has them, as do a few other places. Unfortunately, for me anyway, when the price of an 80% is only a few bucks cheaper than a 100% it kinda loses it's appeal.
AR15 Lower Receiver 7075-T6 Alloy Forging, Military Spec - 1001-D S Arms
These are a better deal if you can handle all the machine work. They are raw forgings.