The most essential element for making brown scapulars can be hard to find
Where I get Mine
I purchase all of my brown woven wool from thrift stores.
The two most fruitful places to look are among the men's suits, and women's skirts.
The most important things are the color (from medium brown to black) and the fabric (100% wool). To confirm that the fabric is 100% wool find the label, these are sewn in a few different places, on a skirt look in the top center of the back of the garment, or along the interior seam near the bottom. In a men's suit the label is often sewn inside the interior coat pocket or along the interior seam.
A wool garment may say something like 100% virgin wool, 100% lambswool, or 100% pure wool, all of these are good for making a scapular, however if the garment is a blend, as in the picture above where the material is 20% Cashmere, do not use it for scapular making.
After I bring the garments home I put them in the clothes washer on HOT to remove sizing and dry cleaning solvents, and to shrink or tighten the fabric. I then put them through a HOT cycle in the dryer to finish shrinking the fibers. By washing and drying the fabrics before use you will minimize the chance that the fibers will warp later when the scapular eventually gets wet (or put through the washer again!)
I then cut up the dry garments along the seams, cutting around zippers and pockets. finally I iron the pieces of fabric and use pinking sheers to cut the pieces into rectangles of a suitable size for making a scapular, usually around 6" x 10" for each half. That is a generous size and it makes handling the material easier when applying embroidery, and folding and sewing the scapular closed.
A two-piece men's suit can provide me with enough material to make around 25 scapulars. I continually look for brown wool at thrift stores so that I can have a variety of materials to choose from, I have wools with different textures and tones, and when I am making a scapular for someone I like to give them the choice of which to use.
Other sources
Online is your next best bet for finding brown wool, Etsy, and Ebay are the two places where I have seen it available, some Etsy vendors will sell as little as 1/8 yd.
If you live in a city with an excellent fabric shop, you can try to find woven wool, likely among the suiting, but I have had difficulty finding dark brown this way, in the city where I live I rarely find 100% wool in fabric shops, and when I do it is typically striped or plaid.