You can use any yarn with the same gauge. The gauge on the Rowan is 9 sts/11 rows = 4" in stockinette stitch. So that makes it a bulky yarn (#6). So look for yarns in that gauge and you're good to go.
Here's a good place to start. Put in the weight you want and fiber if you want as well and see what comes up.
http://www.yarndex.com/home.cfm