something like a handheld unit maybe
If you want something more portable: http://www.amazon.com/VuPoint-Solutions-Portable-Scanner-PDS-ST410-VP/dp/B002R0BFAA
Of course the scanner mouse has its pro's over this but this adds one less step to scanning something quickly.
What I found dumpster diving google.
http://www.amazon.com/VuPoint-Solutions-Portable-Scanner-PDS-ST410-VP/dp/B002R0BFAA
Tempted to buy one.
Not quite a pen, but you should be able to use a portable scanner like this. It will save text as an image, and then there are any number of free downloadable OCR programs you can use to convert the image to text that you can put into a word document