I just submitted my 'Hausarbeit' so I know what you're talking about. But to be honest, checking an essay of ceveral pages is a lot of work and I don't think you'll find anybody who's willing to help you without payment. And I think that's how it sould be! Submitting a paper for university is serious and the grammar and the structure of the sentences should be impeccable. There are professionals who can help you and normally they need two days to check an essay with up to 20 pages (longer for really badly written essays) - which shows you how much work it is and this work should always be rewarded! I always work with http://studi-lektor.de/ because they have high quality employees, I'm always satisfied with the result and they work quite fast. Of course, this applies only if you want to submit high quality essays. If you don't really care about quality and fair payment, well, then it might be that you'll find somebody (under-qualified) who's willing to help. Try discussion platforms for students for example. Good luck!