This is the question I get the most. I'm not going to tell you my particular sources, because I have taken pains to develop good relationships with them, and I'd like to keep it that way.
However, here is where you can start: places that display banners, places that make banners, and places that install banners (they may have a "toss list"). Organizations using banners that have specific dates on them and that cannot be reused. Those might be good places to try. Also Ecofreek, Craigslist, Kijiji, key words being "tarp" and "vinyl banner". Emphasize that you are recycling the material, and express gratitude and try to give them something back, such as samples of what you make with the material. Don't expect them do just hand the material over, either. The raw material costs something like $12/yard. Also they may already be donating them to local schools and the like.
Also, reassure them that you won't use the advertiser's logos and imagery in a recognizable form unless you obtain permission first. And then keep that promise. You could also get sued if you use logos and imagery without permission. Some folks on Etsy have found that out the hard way.
I started this blog to promote recycling of this material, and to share what I've learned. On one hand I'm happy to share; on the other I have spent a lot of time and effort investigating my techniques and I don't want to give all of my secrets away. I'm sure you understand!