According to this website
not stickers, but the pictured metal signs with the wanderer pictogram are on sale for about 20 Fr.
You can even have your personalised direction sign with four destinations and time specifications for about 190 Fr. (p.e. ETH - 20min, Madrid - 19d 10h
You can ask them if they sell the stickers, or the "Wanderweg" signs with crest of the canton, too. Their phone number is 031 370 10 20 but since you don't know German or French well you may prefer to write them a mail in English to firstname.lastname@example.org