Hi buddy, there is not sites that I know selling NS modules, Ebay is where You have more chances of finding one but not by itself
You have to keep an eye for a working module on an ugly case, NS came in many flavors on the 29mm men. early dot display time only, dot time and seconds, dual time with and extra pusher at 8 O'clock, 6 functions "DOW', All are 29mm and interchangeable. almost every watch with the pushers at 2 and 4 O'clock is a NS with the exception of Collins.
someone correct Me if I 'm wrong. You shouldn't pay more than 40 US dollars but aim for less. In me experience You will have not trouble finding one, a lot where made and are very reliable. If You have a problem finding one let me know i can Help. Ismael from california