I don't think they make them. The closest I came was googling "Big XII bumper stickers" and just got a lot of images about Texas, gold-digging and what looked like a dragon standing on top of the symbol of the Deathly Hollows.

https://www.google.com/search?q=BIG+XII+bumper+stickers&biw=1088&bih=733&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjdqd6ty4TOAhUO9GMKHRlOAbcQ_AUICCgD

Not sure what to make of it.

And is the P12's official color Red? Because I couldn't find a green P12 sticker for Oregon fans.

https://www.google.com/search?q=BIG+XII+bumper+stickers&biw=1088&bih=733&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjdqd6ty4TOAhUO9GMKHRlOAbcQ_AUICCgD#tbm=isch&q=Pac+12+bumper+stickers

BTW, I love the BYU flags that pop up when you google "PAC 12 bumper stickers". Hahahah.