Check with influenster or even the diaper companies themselves the diaper companies might send you free diaper samples for testing and review if you post a review on a blog you run.
Other websites might have contests where you can signup to win free diapers for a year.
Other than that if you need free diapers because you can't afford diapers then you might look into a diaper bank where if you qualify you can get free diapers from a diaper bank.