My experience emailing companies for free stuff. I have received free stickers, free samples, free products by simply emailing companies and asking them. Older posts · Anchor In Clothing · What To Write · Brooks.
The second way is to email companies asking for stickers. You will not always get a reply but more often than not it works. Just say something.
Here is a list (I'll be adding more soon) of companies who have been known to give out free stickers by mail (without you having to send an S.A.S.E.) * indicates.