Looks like one of these. lots of fun. http://www.amazon.com/Pilot-Parallel-Calligraphy-Cartridges-90050/dp/B002RJNT46
I went with this pen marketed as a calligraphy product, so hopefully it'll be an okay place to start. The line mostly has positive reviews, and cartridges seem to be relatively inexpensive, so I'm hoping it turns out to be a worthwhile investment.
Cool thanks for letting me know. I keep a bag of broken pens just for doing these kinds of little mods. If you have a target near you most carry a pilot fountain pen with an italic nib that is a little wide for my taste but is a really good bargain. I believe it is called a Plumix, otherwise most Officemax carry the Pilot Varsity which while it is a fountain pen and not a bad one, it isn't italic or at all flexible so things don't look too fancy with it and it isn't at all refillable. Otherwise your probably going to want to look at Amazon or Jetpens. Some Hobby Lobbies and Amazon carry the Pilot Parallel which is a really fun pen and affordable pen