Not sure about in the US. There are the synthetics (Wintec/Bates, Thorowgood) and leather Kent & Masters, Whittaker and Kieffer. A lot of manufacturers use plastic trees for certain models eg. Passier, GFS (Fieldhouse), Jeffries.
Unfortunately the only way to tell for certain is to check manufacturers catalogues. Stores don't like advertising 'plastic tree' in the belief it sounds cheap, whereas its really an improvement over plywood and steel in a lot of cases. But anything with an adjustable gullet will have a plastic tree of some type.
Not sure if that's much help :)