Go to the mansion, and save the game. Speak with Mr. Backlot to figure out what type of Pokemon is in the garden that day. If you need it, capture it. If it is not a Pokemon you need, turn off the power, restart the game, and talk to him again. Repeat this until you get all the desired Pokemon. Pokemon that can be found in the garden are Igglybuff, Cleffa, Clefairy, Ditto, Pichu, Meowth, Happiny, Chansey, Bonsly, Mime Jr., Eevee, Azurill, Marill, Plusle, Minun, and Castform.