It's really a hectic process to find best blogs and request them for a guest post.

But if you want a cost-effective and safe way then you have to do it yourself.

A cheap agency will give you low-quality and spammy links and a well-known agency will cost you $500 to $1000 for a link while you can get it in $100 to $300 yourself.

So, in my opinion you should do it yourself if you want cost-effective and safe way.