Go to any retailer (Big W, Kmart, etc) when they're pulling down the posters for games and such, and ask them for a few, since all they do with them is chuck em out.
I have:
-a Super Mario SUnshine poster
-Super Mario Advance 3: Yoshi's Island poster
-Conker's Bad Fur Day large N64 Box(y'know those large replicas of N64 boxes they used for ads)
-Zelda Oracle of Ages/Seasons large GBC box
-Mario Kart Super Circuit 5-foot high cut-out
-Various posters from Nintendo Gamer
-A very cool Large SSBM character line-up poster from UK's NGC Mag.
-Large Wave Race poster from NGC Mag
So start hassling the retailers. They might say yes, maybe no, but ask.
EDIT: Oh, and at the Pokemon Playoff in Sydney, they had two six-foot Samus cut-outs.

Me, Chris and Lance wanted one each, but, no, they didn't let us.