There is no level requirement for purchasing megaphones, but your character MUST be at level 70 or higher to use a megaphone. This applies to both megaphone types, which are described below.
Channel Megaphone
This megaphone enables everyone in the chat channel to view your message.
Server Megaphone
This megaphone allows every player in the Server Megaphone chat to see your message above the general chats.