I started and ran an agency exactly like you described for several years. At it's peak it was a 7-figure business serving startups and fortune brands. Financial armageddon was unkind to us. Some semblance of it is still here: http://www.singleboundcreative.com/
I left to run product and eventually also marketing for one of my startup clients full-time. We were acquired. I'm about to be loose and will be returning to consulting (while I continue to chip away at a side-project of course).
I'm interested in learning more about what you're looking for, and finding out how I might be of assistance.