I am also working on a free standing display. I found these unico Bases from VKF Renzel while reading Sign and Digital Graphics magazine preferred vendors lists. I have not personally used them but maybe they would work for this application.
Unico Base | VKF Renzel USA Corp.
I did actually find another company that sells these but I asked them for a quote 2 weeks ago, I've emailed and called them twice and they still haven't given me a price so I hesitate on giving them out as a supplier. VKF contacted me right away, gave me a price within an hr and answer all of my questions.At that price it looks like they need some competition.