Transparent Blank Wargame Hex Maps

Chessex sells something they call a Crystal Megamat. It's 34.5" x 48", quite a bit larger than A3. However the hexes are 16mm -- I don't know how big you're looking for -- and are numbered with line-of-sight dots. Still it's the closes thing I've seen.

Deemer Games sells something called a Skirmish Board. It's on plexiglass and is 18" x 18", larger than A3 but not a lot larger if I correctly recall A3 size. It comes with 1" hexes.