Pen for writing kanjis on iphone

Hello,
I use the app Kanjibox and I would like to practice writing kanjis on the touch screen of my iphone 4s. Could someone recommend a good electronic pen for this purpose?
Most electronic pens seem to be made for ipads or to be used for playing games.
Thank you very much for your help
Hannah

Here's a website for amazon for your reference:

http://www.amazon.co.jp/%E3%82%B9%E3%82%BF%E3%82%A4%E3%83%A9%E3%82%B9%...

Great, thank you.

Here's another website for touch pen. This one is designed for iPhone/iPad in particular.
http://global.rakuten.com/en/store/jttonline/item/017848/?s-id=borderl...

Ok great, thank you everybody, I found a good one, now I need to practice ;)