Get a Silhouette or Cricut machine and make your own. Plain sheets of sticker paper are cheap & you can make whatever size/design you want.

(May require some graphic design skills)

I still have some (albeit shrunken) shirts from RedBubble that had some project or Linux logos/texts. Incidentally, they also sell stickers. https://www.redbubble.com/shop?query=linux&iaCode=all-stickers&ref=search_box I would definitely get stickers from a project that helps them out though if that’s your thing. Otherwise, enjoy the stickers. ;)