Reddit mentions of 5 ML Clear Duralar 40 inches wide x 12 feet

Best bang for your buck is to just buy a roll of Duralar. That way you won't be confined to a letter-sized stencil and it's cheaper than buying by the sheet. I would also recommend getting a stencil cutter. It won't be as precise as an xacto because it basically melts the plastic and your line will be slightly wider than a normal cut, but it speeds up your cutting time tremendously and you don't have to change blades on it.
Check your local hobby/craft shops for these. You can sometimes find duralar rolls for cheaper than amazon.