Michael’s and other craft stores that carry art supplies are often a good resource for inexpensive brushes, you just have to know what you need. I use Stila’s brush #7 a lot, it’s good for pressing the color into the lid and the smaller size is good for precision and ensuring the shadow stays in the area where you need it. This brush that I found at Michael’s is called the S Royal Soft Grip SG995 and it’s just the same size and density but with an added gripper on the brush… and it was only $2.00. Take in a photo of a brush you would like, or bring one in that you have already to compare. Be careful, some of these art brushes can run you as much or more as the makeup ones, but 9 times out of 10 you will find exactly what you are looking for at a fraction of the cost.
Jane Iredale Powder Brush against Winsor & Newton Series 240 size 3 $5.00.