where can I find credible websites for plagiarism or academic citation
Academically credible websites will usually have a web address that ends in .gov, .edu., or .org. However, that alone does not attest to the site's complete credibility and so it is usually a good idea to also investgate the background of the author(s). Another good idea is to check if the website you're examining provides references and citations for the information it uses.
Here are some credible websites related to the topic of plagiarism: