For Vinyl, I run:
Avery 3903 EZ and/or 3M IJ35C
For Banners, I run:
UltraFlex JetFlex FL Gloss 13oz. and/or Proveer (our supplier up here) house banner stock.
For posters I use Latex Gloss poster paper 7mil & 9mil supplied by Josh on this forum (BigFish). Best poster paper I've found by far for latex.
Funny thing is by the time I pay shipping and import fees, I end up paying DOUBLE what Josh sells this stuff for, but it is still less than we pay up here for an equivalent type of paper. And I still sell it for a very, very good profit!!