Thank you for your efforts to support the martins. I believe the Dunn nest structures were erected as part of a College of Forestry/USGS research study https://cf.forestry.oregonstate.edu/research/purple-martin-study and I'm sure Dr. Hagar https://directory.forestry.oregonstate.edu/people/hagar-joan (link has all her contact info plus link to her USGS website, too) would be more than happy to connect. In general, we (OSU) don't have supplies of materials such as bird houses, etc. However we can point you to guidance such as the  Purple Martin Conservation Association - They have information about siting, box options (such as excluders for pests and predators) and the also do sell nest boxes, etc., but so do others all the way to Amazon. The wild bird stores locally might have or be able to order them more easily and locally.