They show up occasionally in the usual places. I was lucky to find one locally from a friend of a friend, although it's probably the least desirable variant. Prices vary depending on the variant (reference, color, hand-set) and condition and accessories, but they are often in the $20k range, more or less. Keep in mind that they are tricky to authenticate since the markings (and even the paperwork) can be faked pretty easily. There is a ton of info on the internet about Tudor Marine Nationale Subs if you use google. Here's an old blog post by Ross Povey to get you started.
https://tudorcollector.wordpress.com...ine-nationale/