I saw it at Home Depot, it was with the shelf covering back where they keep teh wallpaper. It was in standard size rolls. I don't know the size of your art so I don't know if they would be wide enough to cover from top to bottom. It wasn't expensive at all, around $5-$6 if I remember correctly.
I'll check and see if anyone is selling this online and edit this message.
EDIT:
A quick Google search yielded pages of crafts which required clear contact paper, but I didn't see any suppliers of the oversize stuff. A more thorough search might turn up suppliers, I just did a quick glance through. A standard roll is 18" wide, I'm not sure if that's wide enough for your needs.
While you are at Home Depot ask the person who works in that department if he knows where you could get an oversize roll (assuming you would need one). He might just have an answer.