You just find books with the solutions. Some like Hubbard and Hubbard's Vector Calculus come with a solutions manual. Some like Knuth's Concrete Mathematics c

You just find books with the solutions. Some like Hubbard and Hubbard's Vector Calculus come with a solutions manual. Some like Knuth's Concrete Mathematics contain full solutions. There's also whole genre of books like Schaum's Problem books and their Outlines which contain thousands of solved problems. And they're quite cheap. For any given math subject X, you can probably search for an "X problem book".