Here is where to buy a pattern/books:
http://www.headcoverings.com/ChildrenOrder.htm
http://www.ozcrafts.com/country16.html
http://www.smoke-fire.com/19th-century-ladies-patterns-1.asp

And here are some online instructions:
http://ourworld.compuserve.com/homepages/trailofthe49ers/bonnet.htm
http://www.deelightful.com/sunbonnet.htm has a towel sunbonnet you maybe could convert