I think that should be Crenellations, not castellations (but of course I could well be wrong). As I understand it the crenellation is the 'gap', the merlon(? not sure i remember that right) is the 'block'. I've done a few castle-type things and am always on the lookout for stuff to use in the next one, but have never come accross any ready-made merlons & crenels (aka crenellations). Sorry.
For some castle type inspiriation, you might like to take a look at this blog:
Tudor Doll Castle