I bought a few rolls when I stopped into a Stained Glass Supply store sevearl years ago. But, you can purchase online from
www.glasscrafters.com I believe their S & H is better than most out there that specialize in online stained glass supplies? But, compare before you buy as I don't know that for sure! They have a $20 minimum order...
A roll is usually between $6-8 for 1/4" wide. It's a LOT. I bought 1/4" and 1/2" for my different projects.
As for tarnishing, and patina effect, yes, that will occur over time. However, if you want to keep it tarnish proof, lay the strip out on a piece of newsprint, and spray with Krylon Clear. That will prevent air from accessing the surface of the copper and tarnishing.