How can I find best freelancer writers for my blog?

There are many on Fiverr and Upwork. The thing which matters is what is your niche and who has expertise as per your requirements.

Therefore, its a good idea to give a trial order to someone and see the results, usually, it takes 7–10 days to see the impact and then you can decide about the next order.

There is no hard and fast rule, we have to trial and test and move forward, good luck.