A method you can try is to use Rock Smash on the breakable boulders on Rout 18, entering the small cave at the bottom of the stairs to reset them. Granted, there are only 3 rocks, but they drop a lot of Stardusts. Not exactly a time efficient method however.
You can also take advantage of the fact that this is the only place to find Shuckle. Makes it worth it until you find one at least.