Models of the Nautilus are few and far between, especially in any decent scale. Custom Replicas MAY have a few of their castings left to offer, but you're talking somewhere around $1800US, I think.
I had to build my own due to lack of funds at the time. Blueprints are out there for the people so motivated to give it a shot themselves.
Let me know if you need the plans and I can email a copy to you.
Bob the Builder
www.rc-sub.com