Free stickers are the easiest to obtain from a company, you can get many free stickers from different companies just by e-mailing them and including your mailing address in the e-mail. You can also get stickers that require a S.A.S.E (Self-Addressed-Stamped-Envelope. You send the S.A.S.E to the company, the companies mailing address will almost always be posted on their contact page of their official website.As long as you are kind, and tell them you are a fan of their product, they will almost always send you some free decals or stickers. The reason companies don't mind sending you some free stickers is because the stickers usually have their brand or one of their products brand on the sticker, so the companies consider giving stickers away as a form of advertisement.
The links with S.A.S.E beside them means they require a ( Self-Addressed-Stamped-Envelope).