@raynbow- Chestnut mushrooms are kind of rare, so finding them in many areas is difficult. If you have a natural food store in your area, you should look there. Since these stores cater to smaller customer bases with specific needs, they are more likely to carry items like chestnut mushrooms.
If you still don't have any luck finding these mushrooms in any stores in your area, you can always ask the manager at your favorite grocery store if he or she can get them in stock for you. Some stores will do this, though the cost may be higher than if it were a product that they regularly keep on the shelves.