For sourcing materials, I usually leave it up to spontaneous visits to flea markets, antique stores, estate sales, etc. This process is much more exciting because I let the material come first and then the design comes after, as I never know what I am going to find. I am particularly drawn to materials that might be overlooked, like fabrics with a heavily worn look, patina, rust spots, holes, etc. I also love using military textiles and feed sacks that have unique stenciling and printed details on them. It has also been helpful to make connections with vintage pickers in my area that know what I look for and help me source materials as well.
Some of my favorite "grails" I have found would be some very rare printed feed sacks, one was purple with a rather hypnotic swirl on it and I had never seen anything like it. I also enjoyed finding deadstock embroidered railroad patches from the estate of a railroad conductor from the 1960s. Also, I love finding any military materials from WW2 that have handwritten names, numbers, and drawings on them.