I'm going to finish out the Jules Verne Week Extravaganza with some ideas on how to get that Victorian future submarine look.
First, my favorite stores for nautical inspired decor are The Brass Binnacle and Newport Nautical Decor. For submarine specific things, check out The Submarine Store.
Ebay, which differs from day to day, often offers scale models of the Nautilus, or plans for making them (would be great framed!) or other surprising things (last week they had a piece of plastic seaweed off of the original DisneyWorld ride).
I've also put together a kaboodle list of things with a submarine or 20,000 feel to them -- highlights of this are a model giant squid and rusty iron floor tiles.