You can use an old sock for fine-grained things like powdered sulphur (wonderful for repelling mosquitoes, just pop the sock against your legs a time or two!).

For bigger granules, use an oversized tea strainer, 10" or so across.

You can get the strainer in most dollar stores.

I've made a duster out of the plastic containers that sugar-free powdered drink mix comes in. the containers are 6-8 inches tall and about 1.5-2 inches in diameter, with a snap on lid. I punch holes in the plastic lid with a nail, fill the container with dust or powder, and snap on the lid. I can either shake the dust over the plants, or I am able to hold the canister upright under the plant's leaves and squeeze the canister to blow dust up onto the underside of leaves. Works great. When it gets old, the plastic cracks; then I make a new one.

Use a 2 inch PVC pipe,choose a length that is comfortalbe to handle, attach a sock or a fine wire/plastic mesh to one end, a c-clamp works well. I've used this set for several years, works well.