Where are the upanishads contained?

Hello, I am interested in reading about Hinduism. I have only read the Bhagavad Gita twice so far. I am desperate to read things like Charaka Samhita, Atharva Veda, and more specifically Sandilya Upanishad and Savitri Upanishad. But I heard that Sandilya Upanishad is a minor one - is this contained in any books? I want to have physical copies.