A lot of stylus pens for iphone are sold.
but you shuld notice the durability of it.
From my experience, the silocon head of it is bad.

One of my recomended stylus pen is one manufactured by adonit.

http://adonit.net/

Or I love the carbon nanotube head,
but I know it only in Japan.

http://www.princeton.co.jp/product/digitalaudio/psatp5.html