I’ve made chocolate curls from thin bars. Each curl comes out as a narrow band of ribbon, as opposed to wider. The important thing is the temperature of the chocolate. Too cold, and it will shave off in brittle flakes. Too warm, and it will start to melt.
You could try tempering two of the thinner chocolate bars together (i.e., partially melt one side of each with a hair dryer, press them together, so that you end up with a thicker bar.) I don’t know if you will be able to see the “seam” once you make the curls, but the good thing is, the mistakes will still be delicious.
You can buy thick blocks of chocolate online at Sweet Celebrations, and Surfas, Inc. (Surface is a restaurant supply store here in Los Angeles—a cook’s paradise!)
Also, Scharffenberger bars are thicker than, say, a Lindt bar. You should be able to find Scharffenberger in well-stocked supermarkets.