Can you only buy ebooks on nooks in canada and usa?

Not if you are trying to buy it using the nook. Some epub ebooks you can buy online and then upload to the nook, but you should get it from an ebookstore that is not Sony or Kobo. If the epub is DRM free, you can buy and upload with no issues no matter where you get it from. Also, there are geographic restrictions, some ebooks can't be bought outside the US.