Clawhammer Supply : Primarily sells in pre-cut kits, but their materials are widely cited as the industry standard for home builds.
eBay : A good source for finding both copper patina or mill finish sheets, allowing for price negotiation with sellers. Key Technical Considerations 22-Gauge Copper Sheet 36" x 96" (3'X8 - Moonshine Still
Metal Supermarkets : Provides several grades of copper sheet that can be cut to exact specifications and picked up at local physical locations.