You can try online platforms like Amazon. They usually have a wide range of 'Haikyuu' light novels available, both new and used. Just make sure to check the seller's ratings and reviews to ensure a good purchase.
I have no definite answer. Sometimes manga series end when the story feels complete to the creator or when publishing circumstances change. So, we'll just have to wait and see.
The Haikyuu manga is indeed over. It had a great run and left a lasting impression on readers. The story came to a satisfying end, tying up loose ends and giving closure to the characters' arcs.
Season 4 of Haikyuu begins approximately at chapter 290. But keep in mind that there might be some variations depending on different translations or editions.