I'm often asked about the pens I use. The one I use most often in the Koh-i-noor Sketch Pen, that was, unfortunately, discontinued about 15 or so years ago. They rarely pop up on eBay, but one is there now.
I don't know how high the bidding will go, but it started with 50 cents.
http://www.ebay.com/itm/koh-i-noor-art- ... 500wt_1118
There were two versions of this pen. One with the larger, yellow barrel is the original model. This sleeker, black one replaced it.
Of course, I don't really endorse this pen as the right one for every artist, just as I don't endorse the quality of this particular pen or the seller.
koh-i-noor pen on eBay
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