Some of the flowers require grafting, others require certain packs. I think snowdrops require the Seasons expansion, for example. You can always check a guide for more specific info:
You can buy packets for each season. Also, set up a holiday with gnome appreciation as one of the activities. Once you're on good terms with the 3 gnomes that show up, they'll drop plenty of packets.
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I don't wanna cheat, I wanna play.
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I cheat and use bb.showhiddenobjects and get them there.