well you can just go to a car accessories shop & ask for car stickers...
but they will be just for show off...
no income...
many guyz do it all the time....
but
For actuall brand marketting, just go to a brand's headoffice & ask for marketting department.
there, wait for some time & then meet the marketting executive or marketing manager & ask for their brand stickers for their BRAND MARKETING....
The actuall info for every company may be different from others...
by the way, it is good source of income in some countries...