Where can I find new map ideas for Settlers of Catan Seafarers?
There are no official "sequels" but there is some stuff you can find online. 

CatanMaps.com is the largest library that I've found. They have about 50 Catan maps or scenarios (mostly for Seafarers), and they add to it regularly. The collection includes info on setting up official scenarios that didn't make it into the current handbook (Greater Catan, Enchanted Land, Desert Raiders, etc) as well as fun adaptations like Middle Earth, Westeros, World Map, etc.

EDIT: The site seems to have closed but the maps are saved on their Facebook page.

MenzGaming.com is another site with about a dozen Seafarers scenarios. I have only tried the Race to Cities of Gold one, but they all look fun.