We've posted a video review of one of Montegrappa's officially-licensed Game of Thrones pens. Montegrappa is a famous name in Italy, a maker of fine pens and other luxury items, and their licensing Game of Thrones has led to four (now five) beautiful pens.
The Stark pen featured above -- a fountain pen with a medium nib; other options include fine and broad nibs, or rollerball or ballpoint versions -- is one of the initial four pens which each take their inspiration from a different Great House. The fifth, and newest pen, is a high-end limited edition in solid sterling silver or even 18 karat gold, but those run anywhere from 3,200 to 52,000(!) euros.
This pen came to us by way of Amazon.com, via a Madrid-based authorized dealer named Iguana Sell (noting them down because they did an amazing job with prompt shipping and really good packaging), but you'll find most of the varieties of the pens available from a number of sellers besides them, some at even lower prices:
Of course, the HBO Store (both the US and EU varieties) also carries the pens as well, and sometimes you'll find special deals on all their Game of Thrones products.