The sachet can be found two different ways in both sword and shield. First way is through the Battle Cafés. You can battle the café master in each cafe once a day and if you win he will randomly award you one of 13 items; one of them is the sachet. (source)
There are three battle cafés: the first one is in Motostoke past the Pokemon center in the lower middle part of the city. The second one is in Hammerlocke to the left of the middle Pokemon center; the café will be between the boutique and salon. The third one is in Wyndon along the street to the left of the Pokemon center. (source)
If you want a less random way to get the sachet, you can buy it. The sachet can be bought from the league staff member inside the middle Pokemon center in Hammerlocke for 10 BP. (source)
