Aurora made a very nice Sealab III with an interior, but it's been out of production since the 1970's. Worse yet, it was used as a donor kit for a lot of Star Wars studio models so demand is high. Here's a good rundown on the kit:
http://www.greysteele.com/models/sealab.htm
Any of the Sealabs are eminently scratchable. For something a little more science-fictiony, there are some Japanese space base and gundam base/factory/fortress kits that could play the part.
Andrew