How to find a co-author? I am working on a manuscript and need some collaborations in the part which is out of my expertise. I personally don't know anyone in this field.

It will be a good paper to be published in a top journal. How can I find a co-author to help with this?

Is it OK if I send my manuscript to some academics in the field and invite them to collaborate, or it is an odd invitation?
A few possible things you may do for a prospective collaboration:


Look for authors who published in the field which you wish to publish in (citing from this post). You ought to have researched on a set of papers from which you would have developed your idea or an enhancement to. The authors from those papers may guide you.
Contact in sites that facilitate collaboration and discussion like ResearchGate.net and Academia.edu. There are many who have collaborated this way and become from strangers to research associates.
Mail invitation; this may or may not work. A discussed in the comments, your mail may be tossed by a spam filter or even be ignored. But it has worked before for some people.


Having stated above, getting reliable co-authors, may not be so easy. This is one of the reasons why professional research ought to be conducted with a supervisor/advisor or a mentor who is established in the field. They ought to guide you along these lines.