Some other artists are approached with opportunities while they work as a mangaka’s assistant. Noda Shigeru (Dr. Kenji Morohashi, Kakesen) made his debut that way, as an editor liked his drawing and offered him the chance to pair with an established writer. Or, alternatively you could be an author recruited to write (but not draw) a manga based on the success of your novels, like Kazuma Kamachi (To aru Kagaku no Railgun), or a celebrity asked for a collaboration, like Shokotan (Noko no Koi).