Physical Vignette (Sticker):
Gas stations (petrol stations) — especially those near the border or along main roads.
Post offices in Switzerland — official postal outlets sell the sticker vignette.
Customs / border-crossing posts — at the border when entering Switzerland you can usually buy the vignette.
Digital Vignette (E-Vignette):
You can conveniently purchase the yearly e-vignette for Switzerland here.
Related articles:
What is the Swiss motorway vignette?
How much does the Swiss Motorway Vignette cost?
How long is the Swiss motorway vignette valid?
What happens if I drive without a Swiss vignette?
Can I automatically renew the e-vignette each year?