We use Dibond brand (Alcan Composite) 4050x2050mm sheets here; can get them in 4, 5 & 6mm thicknesses I believe. You may also be able to get the sheets in Alucobond (Alcan) as well. Both are European made products and really good quality. Our main shop sign is made of 2 sheets side by side, measuring 8000x2000mm on an ally frame which we put together in the shop and lifted with a small crane into place.
Be careful though, they are really big sheets to handle - you need to make sure you have plenty of room to applicate from both sides, and plenty of hands to install them.
Cheers,
Neil.