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Edited 7/13/23 to include newer material supplier: https://www.onlinelabels.com/
I strongly recommend paying the extra money for the "Aggressive White" material rather than the Standard Matte. It handles uneven, dirty, and cold surfaces that the Standard Matte can't.

They also have an Overstock section which has deep discounts on various materials including premium materials such as clears, metallics, etc. https://www.onlinelabels.com/products/overstock
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These are home made; printed out.

I get my label stock from Next Day Labels because the quality's good, it's USA made, and the price is really sweet (especially in the case quantity.) You can also pick up label stock in smaller quantities at your local office supply store, but you're going to pay significantly more.
The two sizes used in the post you're replying to are these half and quarter sheets:
https://www.qualityforms.com/catalog/product/view/id/2073/s/8-1-2-x-11-2-labels-per-sheet-8-5-x-11-pack/category/86/
https://www.qualityforms.com/catalog/product/view/id/2575/s/8-1-2-x-11-4-labels-per-sheet-4-25-x-5-5-each-label-pack/category/86/

They also have templates for all the sizes ,which makes layout and design much easier, especially for the novice (like me).
http://nextdaylabels.com/templates.html

Stickers are often found on: Shopping carts, Gas pumps, ATM machines, Entrance and exit doors, Fast food drive thrus, Grocery store checkout areas, USPS drop boxes, Park benches, Bathroom stalls, Hand sanitizing stations...

If you would like to create or print your own stickers, here are some resources to get you started!

RESOURCES
Image editors:
>>> http://photopea.com (online Photoshop clone, free, easy)
>>> https://www.gimp.org/downloads/ (desktop, multi-platform, free)

Templates:
>>>Avery labels templates: https://www.avery.com/templates/category/address-~-shipping-labels (widely available from office supply stores)
>>>Quality forms templates: http://nextdaylabels.com/templates.html (see below for bulk labels)

Bulk labels:
>>>Rectangles
1x https://www.qualityforms.com/8-1-2-x-11-1-label-per-sheet-8-5-x-11-item-332155-332155c.html
2x https://www.qualityforms.com/8-1-2-x-11-2-labels-per-sheet-8-5-x-11.html
4x https://www.qualityforms.com/8-1-2-x-11-4-labels-per-sheet-4-x-5.html
6x https://www.qualityforms.com/8-1-2-x-11-6-labels-per-sheet-4-x-3-3.html
10x https://www.qualityforms.com/8-1-2-x-11-10-labels-per-sheet-4-x-2-331650ream.html
15x https://www.qualityforms.com/8-1-2-x-11-15-labels-per-sheet-2-11-16-x-2.html
20x https://www.qualityforms.com/8-1-2-x-11-20-labels-per-sheet-4-x-1.html
>>>Circles:
6x https://www.qualityforms.com/8-1-2-x-11-6-labels-per-sheet-3-333-x-3-333.html
24x https://www.qualityforms.com/8-1-2-x-11-24-labels-per-sheet-1-625-x-1-625.html

*Number is stickers/page. For example, 10x 250 sheets is 2,500 stickers. Don't forget to share.

Commercial printing:
https://www.printusmaximus.com
https://stickermule.com

Note: We encourage you to adopt strategic thinking, and craft effective messaging around issues that you believe are important. Individual contributions do not necessarily reflect the position of the group, individual members, or members as a whole. Welcome to the Sticker Wars.

Additional resources are welcome in the comments.