Including universities, scientific research institutions and local libraries (such as the National Library and Shanghai Library).

Libraries generally buy a large number of periodical papers databases for internal staff to use for free. There are Wanfang and VIP in Chinese and ProQuest and Springer in foreign languages.

Note 1: Libraries generally provide document retrieval services, but some fees may be charged. Note 2: Some databases can be accessed through IP proxy, but this is illegal.

2) Use search engines

Generally speaking, it is better to search English with Google because it contains a large number of foreign websites.

Baidu is mainly aimed at China. If your topic is especially oriented to China, then Baidu is also a good choice.

Since official documents generally use pdf, to search pdf documents, just add a filetype:pdf after the keyword.

In addition to the web mode, you can also search in Google Academic or Baidu documents, which complement each other.

3) Ask the author of the paper or netizens for help.

Send an email directly to the author of the paper, and there is still a certain chance of getting a reply. In addition, some authors will put their papers on their personal websites.

As for online help, you can enter the forums of colleges and universities or some academic websites.