You can get LN2 at a specialized gas dealer. You usually find those dealers in industrial areas, and usually they accept selling to individuals.
If you come with your own bottle it's *way* cheaper, all they have to do is refill it - the bottle must be certified for LN2 though.
What they said is true, you'll prolly spend cash for an afternoon of extreme overclocking fun. Dont do it unprepared...
About insulation what's true for pelts still stands here. Now it depends if you want a total immersion (LN2 is non conductive) or just chill the core (same system as pelts). You can also chill 3M FluorInert ($1000 per liter) with a closed LN2 loop, and use total immersion in the fluorinert... Those are just examples of what has been done, use your imagination
[edit] ha if you do it dont forget the digi cam ! Share the fun with us !