Look for the three dimensions of inbound link resources: authority, relevance, and reach. Method: (1) Google direct search, pdf sharing, blog builder, bookmark sharing, similar blogger site, etc., there will be many articles listing relevant resources, then check if they are available. (2) Search Google for keywords related to industry or product, check search results, and find external linking resources available on non-trade websites. (3) Advanced Google search, for example, search the color contacts forum website: color contacts inurl:forum. (4) Search external link resources of competitor sites, semrush and ahref can check competitor backlinks, you can see link type, resource weight, traffic brought by links, etc., download them and categorize and organize them. Check out the resources that can be submitted to the external chain and sign up to start posting external chain links.