I usually buy aonori from a Japanese grocery store like Marukai or Mitsuwa. Amazon has them too:

https://www.amazon.com/Lettuce-Japanese-Takoyaki-Yakisoba-Seasoning/dp/B0B4RBMPP7/

https://www.amazon.com/Otafuku-Aonori-Flakes-Seaweed-0-21oz/dp/B071ZKZN1X/