Here is one supply house that has vellum (thin, tough paper that erases well) sheets in larger sizes. I only noticed that they had 8x8 and 10x10 to the inch grids, but I didn't read the whole thing carefully.
The 8x8 grids are nice.
http://www.nationwidedrafting.com/st...ingpapers.html
There are other suppliers around, too.