I am finally able to post a comment:
CustomPapers.com: very expensive (too much, probably) but provides quite a good service, even though I think they should guarantee one revision.
Iresearchpapers: horrible; I guess they use very young students with absolutely no expertise in the subject. I obtained to have the paper rewritten, but results remained poor and the client service refused to allow me for a new revision (even though on their website they say they provide endless revisions). Stay away: it is a bit cheaper than others, but the quality level is...awful.