You don't want "foundation" paper, you want paper-piecing paper. I learned this the hard way. I bought some June Tailor foundation paper at Joann's and discovered that it does not tear. It's meant to be left permanently in the quilt. You want to make sure the package says it's paper-piecing paper and tears easily.
Having said that, I prefer to use vellum that I get from office supply stores. Carol Doak also sells her own brand of paper that is good for tearing away.