I cheat and use my brother who has an English degree in technical writing. I also have friends who majored in creative writing, but they're so busy with their own projects that they hardly have time for mine, so we don't really use eachother, you know?
But truly, outside of family and friends, I think a lot of people would be willing to proofread your chapters for you if you did offer a rate, even if it was a low rate. I'm not sure what the going rate is, but if you think about $ per hour, make sure it's something reasonable, since a proofreader, even for a technical writer, will get VERY nitty gritty, and it does take them a while to do.