Michael Atkins offers some excellent advice on his Seattle Trademark Lawyer blog about finding expert witnesses for trademark cases.
One popular online source is the online expert witness directory JurisPro. Other expert witness websites listing the credentials of various trademark experts include ALMExperts.com, expertwitnesses.com, and Intota.com.
Of course, one of the more interesting "trademark experts" out there touts his credentials via YouTube (link here). Of course, before you hire him, you may want to check out this wikipedia writeup or the string of posts about this expert on The TTABlog®.