The Lawn Ninjas were originally designed as garden-guarding gadgets by Megacorp but were found to be too wild. Lawn Ninjas appeared as enemies in Up Your Arsenal on the Thrann Asteroid as part of the defense system for Captain Qwark's hideout, most likely available on the market now. It's also possible that Megacorp merely gave Qwark the lawn ninjas to thank him for testing some of their merchandise.
They also appeared on a card in Deadlocked in the Exterminator trading Cards cutscene. In Ratchet & Clank Future: Tools of Destruction, they were mentioned by Captain Qwark in the arena on planet Mukow. Much like the Tornado Launcher, GrummelNet also sells Lawn Ninjas, however it doesn't produce them.