I am a amateur biotech enthusiast not affiliated with any university or institution. I recently wrote a Review article about animals that are (more) resistant to cancer (blind mole rats, naked mole rats, elephants, bowhead whales, etc). I want to publish it in a peer-reviewed oncology journal, but before this, where can I find someone to proofread this article for free?
If you're interested in submitting the article to the scientific community then it does not only need proof reading but also specialist editing. The scientific community relies on the process of peer review to ensure that the information published is factually accurate and contributes to the spectrum of accumulated knowledge.
If you feel that your review article does fit these criteria then it may be worth getting in touch with someone at your local university and run it by them. They'll be able to give you some hands-on guidance with your article.
You might want to start by using Grammarly it's free and it will help you clean up passive voice, proper structure, etc... the paid upgrade is very reasonable too.
If that does not work, you will definitely get a "proof-reading" when you submit for peer review. I would suggest that you start by submitting your work for presentation at an appropriate academic conference. If your abstract is accepted you will be granted the chance to present your work to others interested in the same area. Then foster some relationships and ask if they would be willing to help you out. In some cases they might ask for authorship on the paper so be prepared for that sort of answer.