I got mine from Art Friend, very big piece. Maybe 4ft x 8ft (can't remember but its the biggest available). Each piece was around S$30-50 (can't remember again )
Last I looked, they no longer had that super big piece.
for Artfriend, can try to ask them if they can give you bigger size... the sizes you see on display are the ones they pre-cut for "mass consumption"... they might have larger un-cut pieces in storage... worth a try...