If you are in need of more diamonds & coins to finish upgrades or buy items in Hay Day, the only way to do that is through the in-game shop which runs via iTunes and Google Play.
There are several third party websites who offer in game currency for Hay Day. Please notice that using sites like these is against our terms of service, and we reserve the right to take actions against accounts that do not follow our terms of service. Often these third parties will ask for your Game Center credentials. If you share your Game Center ID and password with somebody, this person might be able to access your game through Game Center.