You can order it from Amazon and many other online bookstores, as well as many local bead and book stores. You'll find a link to it at Amazon to the right.
Otherwise, it's available directly from Kalmbach Publishing
Sun Country Gems
You can also check the following book searches and shops for current availability and prices:
Due to an ommission in the book, Rio Grande was left off the resource list in the back of the book. They will be producing and selling the filigree wire. They will hopefully have it in stock by the end of September/early October. Rio also supplies a wide variety of silver wire and sheet, findings, gems, beads, tools and display material. To order a Rio Catalog, visit http://www.riogrande.com . You do not need a wholesale ID to order from Rio.
Due to another glitch, the filigree wire is not listed in the current Rio Catalog. The wire in the catalog is a twisted square wire. This should have been flattened as well. If you cannot wait for the actual finished wire to come in, you can order the twisted square wire and roll or hammer it flat yourself. When the filigree wire does arrive, the stock number will be: #100-831