Plant trees of fruits not native to your town, then sell the fruit (and keep planting more, too). Also, pick up seashells on the beach and sell them. Do chores for neighbors and sell stuff they give you that you don't want. Catch fish and sell 'em. Sell fossils you've received back from the Farway Museum.
If you're REALLY, REALLY desperate, you can always use the 100 turnips code and sell 'em. You can only say up to 3 codes per day, though. (Yes, I admit, I now cheat by using the 100 turnips code whenever the price gets really high. Thanks to CW for that little tip...) Ahhhhhhh!!! I'm corrupted! Ah well, it was bound to happen sooner or later. :)
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If things don''t go your way, just keep complaining until your dreams come true.