Roof flashing might suit your needs. You can buy rolls of it at Lowe's or Home Depot.

If you haven't ordered online yet, check with Ornamentea in Raleigh. They often have sheet copper in various thicknesses. Not sure if they'll have the 8" width though.

Roofers use what they call 16 oz copper, 16 oz per square foot. That's equivilent to 24 ga, 0.0216". It is typically alloy 110 (99.9% pure copper) and is annealed dead soft. It is VERY soft and easily formed. Go to a large roofing company and visit their metal shop, they will likely give you a scrap "drop" that size or charge you a small amount for it. There is very little practical difference between alloy 110 and pure copper unless you are using it for electroforming or plating anodes.
I don't like refuting other's information but this is a topic I am highly familiar with: When annealing copper unless you want to reduce the thickness and surface finish you need to coat all the surface with flux. The best easily available flux for this is boric acid (available at any good drug store) dissolved until saturated in denatured alcohol (wood alcohol, camp stove fuel, shellac thinner). It doesn't take a lot of BA to saturate the alcohol, as long as there is boric acid powder in the bottom of the container the solution is saturated. Clean the metal with soft scrub and rinse, apply the flux liquid with a clean brush and allow to dry or light off the alcohol to form a white fine powdery coating. Heat gently at first until the flux melts and flows over the surface in a glassy coating (in dim light the copper should be glowing a very slight red). As long as you keep the flame moving it's unlikely you'll melt the copper. You do not have to reach annealing temp on the entire piece all at once, but all parts should be brought to a dull red some time in the annealing process. Air cooling will produce "dead soft" temper, the softest state. Residual flux can be dissolved by soaking in hot water or speeded up by "pickling" in a hot solution of water and sodium acid sulfate (available at any pool supply center as "ph down" water adjustment powder). Dissolve about 1/4 c. /gal of water. Use in a plastic or glass container. The solution is a mild acid, not dangerously so, the temp of the water is more likely to cause injury, but keep off clothing, wear eye protection, and wash off any skin contact at the end of the process, especially if redness or itching is noticed. Rinse the piece and hands with clear water after pickling.
Do not allow any ferrous metal to enter the solution while the copper is in there, use plastic or wood tools to handle the copper. The annealed and pickled copper will be a dull fleshy pink color.

LOLz at your comment about the scrap booking store staring you down. At the apartment I used to live at, it was an older building. We had old copper decals outside the door frames. Of course, once copper became super valuable, they all got stolen. The apartment complex tried to CHARGE US for it. Saying we stole it. Duh. Luckily, we had renters insurance...we made a burglary claim.

The lengths to which people go for copper...