it's true that it's publishers rather than writers who usually need proof-readers. Nonetheless, I recently proof-read a book for a friend after it had been through the publisher's own reader, and there were still plenty of errors in it. This is no surprise to me; the standard of editing and proof-reading at publishers today is pretty awful. So your offer sounds like a useful one.
You could offer your services to publishers, who certainly need someone capable of doing it. I've heard of people finding part-time work this way, but I don't know how many of the publishers would be interested.