I agree with you, and Nina's web site is quite informative. There is another agency - Gloria, but unfortunately their services aren't of high standard.
There is also a woman in Sebenik, she used to do weddings in Mexico and Bahamas and doesn't limit herself to only one town. The problem is, I heard about her through a friend's friend who met her when she lived in London and have yet to find her contact details. If anyone else knows more info, I'd appreciate if you post it here. We are getting married later this summer and even though Dubrovnik is an amazing town, we would prefer some other location. Thank you, Claire.