There truly are many sites where you can find writers, you could also try to find someone at a local college to write for your blog. Some people reach out to other bloggers whose style and quality they like and ask them to write posts for them.
Be warned that you can use cheaper sites and cheaper writers but to get the advertised prices, your post will be extremely short and may not meet your personal taste or quality standards. Further, on sites like Fiverr, you could be nickel and dimed to death because writing a quality piece of at least 700 words or more does not often occur (if ever) for just $5 and the writer will have to add on additional fees to make their efforts worthwhile.
You may want to check into Textbroker's 5 star writers or use a site such as Upwork. Just be sure to clearly set your requirements and pay more for writers, do not expect to be happy will the cheapest work. When you find a writer you like whose work you're happy with, do all you can to keep them happy as well.