Thank you for the comment, Mike.
Oak tag is a heavier, stiffer type of paper. It is mostly used as file folders to put inside of the hanging folders in file cabinets. They can be bought at Staples or any other office supply store. Here is a pic to help.
As can be seen, this one has been cut up & used. Sorry that I do not have a complete folder. The last few folders I have are all cut up & this was the biggest piece I currently have. This has been my favorite material for years for making templates.
Hope this helps, Mike.