I recently purchased a fabric marking pencil that looks and works like a typical mechanical pencil. It has several different lead colors, but I purchased the white lead along with it. I absolutely love it as it marks with a finer line and does not get a dull point that sometimes happens with other marking pencils. There are several different manufacturers, but I sent away for this one:
http://www.sewline-product.com/Sewline_Fabric_Pencil