The great thing about knitting shawls is being able to substitute yarns for things you have on hand. If you are wanting a drape that is similar to what is portrayed in a certain pattern, go with the patterns recommended yarn or one with similar fiber content. You can change yarn weight but might need to go down a needle size. It will change the size of the shawl. 

Here is a link to a blog post by Marie about yarn substitution:  

And one about shawls: