1. China HowNet: China HowNet is the core representative of authoritative literature sources, and most universities have cooperation with China HowNet. You can download documents for free on campus, and you need to pay off campus. You can search the required literature by keywords, article titles, authors, etc.

2. Wan Fang Data: It integrates hundreds of millions of global high-quality academic resources, and integrates more than ten kinds of resources such as periodicals, degrees, conferences, scientific reports, patents and videos, covering a wide range. Literature retrieval can be based on the title, author and keywords.

3. School Library: Generally speaking, good universities and school libraries can find reference materials related to your thesis. If you really can't find it, you can search on the paper website according to the first two methods.