UL Certified: up to 6oz.
Most multilayer PCB laminate is made using Electro-Deposited (ED) copper. This copper foil is created by plating copper sulfate to a metal drum and peeling off the resulting copper foil. Since plating is a function of time and amps, the higher the desired copper weight, the slower the output. While there may be additional limitations, it seems that as an industry our copper clad laminate suppliers top out at providing 6 ounce copper clad laminate as both part of their UL cards as well as physical offerings. Saturn is certified for up to 6 ounce starting copper on inner and outer layers.
Von Roll: up to 10-12oz.
We are still trying to dig up the information, but back in the 90's and early 2000's, auto suppliers Lear and United Technologies had high running programs using non-plated through hole designs and 10-12 ounce copper laminate. There was one supplier in the world at that time for this raw material out of Spain. Not sure how popular this material is currently but we will add more information as it becomes available.
Fabricator-made: up to 20oz.
Over the years we have been approached with ever-increasing copper weight demands; as a result, we often found it necessary to make our own raw materials. So far we have maxed out at 20 ounces, but the Designer's imagination is the limit. Give us a shout if you are looking for help on extreme copper PCB sourcing - we're glad to help.