Stained glass widow people use varous types of copper and brass tape / foil in their work. A widely used suppler of this tape is Venture [ http://www.venturetape.com ]. They may be able to help.
We're not sure how your customer plans to use this tape, but we'd try to steer them away from soldering to the tape. Brass is an alloy of copper and zinc. Zinc from the brass diffuses into the tin causing solderability problems. So, you need a barrier plate between the brass and the tin. Use a flash of copper or nickel-plating between the brass and tin. Nickel has a longer shelf life, but copper is easier to control, so we suggest you use copper.