The BBC tend not to licence images from their show (although I have seen some Dr Who fabric some years ago). Merlin is a relatively new show, and the images from it wouldn't yet be released for licence (its all to do with copyrighting and public service broadcasting, and the terms of the broadcast licence issued to the BBC - no I don't understand it either!).
There are a few fabric manufacturers that have dragons, stars, etc in their catalogue, but nothing specifically "Merlin-y" or "Arthurian". However, I will keep my eyes open on this one - my daughter (17) loves the series, especially Colin Morgan.
The only fabrics I have found are Merlins Dragons - Quilting Treasures - five different designs (allover, wizard tools, dragons, wavy stars, and a printed panel). You may be able to get these as an import, or try
http://www.jbquiltingfabrics.co.uk/s...er-p-5528.html Also try
http://www.ebay.co.uk/itm/Merlin-Wiz...-/390352913860 - this trader has co-ordinating fabric and is a well known and recognised trader, and does ship to the USA.
Sorry I can't help any further than that. Hope this helps.