It seems that Pegasus Publishing has a handful of the officially licensed A Song of Ice and Fire t-shirts available, which have been hard to come by these last years. We’ve verified with Parris McBride of Mystic Spiral Traders, who holds the license to produce such branded apparel, and she states that Pegasus Publishing is her convention retailer and that they must have found some old stock. Perusing the House Stark and House Targaryen listings they have, size options and quantities are very limited.
Thanks to Russian fansite 7 Kingdoms for pointing this out.