I usually do a search on eBay for decals. As GM and the others have noted, look for "Balkenkreuz", not "Balkankreuz". The name comes from the root "Balke", which is a wooden beam. It's related to "balcony"-think of a platform on wooden beams jutting from the side of the building.
EagleCal might have decal sets you can use, too. I've used their sets in the past, and the decals were easy to work with, settled down easily. Their website is here:
http://www.eagle-editions.com/aircraft-decals-for-the-modeler