Stampin Up has a fantasy set that has a nice 3x4 castle stamp in it. I also have a 5x6 castle stamp. I can't clearly read the name, so this is a guess: Tomuchfun Rubberstamps East Lansing, MI @ Greg Dearth.
I recommend going to a local rubber stamp store and asking to see some catalogs. Most stores will be happy to special order stamps for you.
Michaels also has a castle stamp.