Where do ideas come from?
This perennial question has both many, many answers, and none. Possibly the most honest answer is "I don't know, but thank heavens they do."
This whole question, time-honoured in its antiquity, is also made problematic by the concept that all an author needs is ideas. But in actual fact, the seed idea is often not the most important thing. It is just a tiny spark of possibility, and usually authors have hundreds, thousands, even tens of thousands of such sparks lurking in their minds. You have to take that seed idea, and add other ideas, and grow the ideas into words and paragraphs and chapters and make a story, which is something that is much more than an idea, or indeed the sum of all the ideas that went into its making.
You need more than an idea to write a story, you need lots of ideas, and then you need to work with them, the work probably being more important in the long run than the initial idea. Sometimes the work has to go on for a long time, and quite likely the original spark will be abandoned and some other idea will becoming the driving force.
But even though ideas are not everything, they clearly are a vital part of the process.
So where do they come from?
See above. No one really knows, and people find their ideas in many different ways. But there are some things that have been proven to help the creation of ideas and generally boost creativity. (Not drugs or alcohol. Despite some outliers they have assisted far more authors to forget their ideas than create them in the first place. Brief note on this http://www.independent.co.uk/news/science/drunk-writers-were-better-sober-says-psychiatrist-2010053.html)
One of the most important is simply to be inspired by other people's creativity, often I think particularly in arts other than your own. Go to a concert, see a play, opera, ballet, a musical show. Watch a great film. Read a truly great poem, a novel or biography or work of non-fiction. Watch a documentary on something you've never even heard about. Visit an art gallery or museum. Observe glassblowers, potters, painters, artists at work with their hands and eyes and minds.
Another source of inspiration is the natural world, either in itself or as it interacts with human life in a city or town. Lie on a grassy hill, a deckchair, or your garage roof and watch the clouds move across the sky. Go singing in the rain, stamp in the puddles, watch the water go down the drains. Swim in a river, a lake, the sea. Climb a hill or a mountain or just a tree. Watch animals in the wild, birds in the back garden, rats by the railway station. Find somewhere totally quiet, if you can, and listen to the wind.
Do just some of that, and the ideas will come.