You might try looking on major online bookstores like Amazon. They often have a wide range of manga available, including Nana.
I'd suggest checking out specialized comic book stores. Some of them have imported manga and could have the Nana box set in English. Also, don't forget to search on eBay; sometimes you can find rare or hard-to-find items there.
You could start your search at local libraries. Sometimes they have manga collections and might have the Nana box set in English. Another option is to look for fan forums or communities related to manga. People there might be able to point you in the right direction or even have suggestions for specific sellers who offer it.
Look to Chapter 3 for the clearest answer. It begins with Qiao was still annoyed at the attitude of this unknown girl when she heard..., keeps building through She's actually Miss Jiang Li from the Jiang Family?!, and lands on Jiang Yan, forced into a marriage with someone he did not love, had....
Chapter 739, "Not Paying Debts, Shameless!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event.
Chapter 804, "Still causing trouble?", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event. The chapter turns that into something concrete by moving from He never imagined he would be scooped up like this!
Look to Chapter 8 for the clearest answer. It begins with The resentment that had barely calmed among the crowd surged back two..., keeps building through Some of them were relatives of the Jiang Family, who felt it improper to..., and lands on Other people with a handsome and charming appearance.
Chapter 824, "Qixing Tower", is the clearest early answer because it turns Yu's situation into a concrete plot event. It opens with Just as he was feeling alarmed and drenched in sweat, Mr. and closes on Yu Qing got up and pulled Jiang Li along..
Chapter 36 is the strongest chapter-level answer because it starts from But soon, people began to question the authenticity of Jiang Li's bare... and follows through on After swallowing the food in his mouth, he hurriedly took his megaphone....
Chapter 17 is the strongest chapter-level answer because it starts from Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and... and follows through on I've been a mother for more than ten years but never tasted anything made....
Chapter 737, "Shu Yan in Trouble!", is the clearest early answer because it turns Sister Li's situation into a concrete plot event. It opens with South City Film and Television Shooting Base. and closes on Jiang Li was secretly cautious, about to search for news about Ye....
Chapter 830, "Pinky Promise for 100 Years", is the clearest early answer because it turns Pinky Promise's situation into a concrete plot event. The chapter gives readers a real answer by opening on At eight o'clock in the evening, Jiang Li arrived home.
If you want the clearest chapter reference, go to Chapter 26. Early on, Shen Lan was bent over processing a large stack of press briefs on her...; later, the chapter leans on She was nearly forty years old, but because she took good care of herself....