Have you tried looking on the Ravelry.com website? It is a free-to-join website for knitters and crocheters that enables you to store information about your projects, patterns, tools, etc. It also has hundreds of free patterns you can download as well as patterns you can buy and download. In addition, it has a very
extensive yarn database, which you can search through by yarn weight, fiber content (e.g., acrylic, alpaca, etc.), and other criteria. Just sign-up and go to the Yarn tab and enter the yarn recommended by the pattern. On that yarn's page, click the little round
symbol at the end of the Weight field to see a chart that will give you the Wraps per Inch (WPI) for the recommended yarn. That may help you find a yarn to substitute. This is a link (http://www.nancysknitknacks.com/wpi_tool.htm
) to a WPI tool. The page lists the dimensions of the WPI tool (just scroll down the page a little) so you can make one of your own if necessary.