Do you mean a proof reader - one who reads the proofs, or a copy editor - one who makes sure you have no typos and glaring errors before you submit your final, polished manuscript to agent or publisher? There's a difference.
The Society for Editors and Proofreaders gives a chart of suggested rates:
You can of course find people who will do it for less than they suggest, but it gives you an idea of the differences of the services, and you'll see that a complete and thorough edit is a lot more involved than a copy edit, and that's more than a proofread (which should only be picking up the last of the problems after the others have done their work, and anything arising from the setting up of the book itself).
Be sure which service you want before you go looking for such a person.
Most literary consultancies will offer proof-reading services. I've also seen adverts in the back of Writing Magazine. Of course, you will have to pay. Each agent specialises in one or several genres. Invest in a copy of the Writer's and Artists Yearbook to find out who represents what.