In New england, we have a product called crusher run that is basically very well crushed stone with no particles larger than about 1/4 inch and a lot of fine granular particles. If you have something like that in your area, use that as your base. It compacts to almost the consistency of concrete with just a little moisture, and makes a very hard and durable base for flagstone, bluestone, brick, and other unit pavers. You could also use stone dust, but I’d go with the crusher run, and use stone dust to fill the joints between the stones. The final appearance of the project will be more in keeping with flagstones set in and on concrete than having sand or another earthem product between the stones.