99designs. I have used them for half dozen projects and not had an issue. Yes I know other graphic designers may not be happy about using me recommending them, but I've been happy with them.
Also, while the name says 99designs and they used to promote that you could get a design for $99, be prepared to spend anywhere from $150-$300 for most logos. If you document it out in the scope that you need a social media kit, they'll add that in too.