Log in to official website, China National Intellectual Property Administration, and select the "patent search" option on the right side of the home page to enter the search page.
There are many ways to search, and it is recommended to use "abstract" search, which has a wide range and can get the most alternative projects.
After seeing your favorite project, check its legal status and make sure it has expired and can be used.
Note that it is best to observe the patents that have expired recently for a period of time, and don't make your investment fail because the obligee requests to restore the rights.