My first question is Why do you *think* your UI/UX needs updating? Now, I am not suggesting it doesn't, but wanting something and needing something are two separate things. So hopefully you are taking a holistic approach, looking at your user behavior patterns, and identifying possible issues to improve upon.

As far as finding the right people. I would recommend either looking at small agencies (a google search is a good place to start) or using LinkedIn to find individuals to talk/interview.