I work for a large printing company, so I'm quite familiar with paper. What you are looking for is called UV Ultra II made by Neenah paper. They have a website, here is the link.
http://www.neenahpaper.com/OfficeHom...dex.asp?ft=OHH
But I don't see where they sell vellum in 12 x 12 on the website.
Other similar products are Chartham Translucents made by Reich paper and Canson Satin made by Arjo Wiggins. Maybe if you do some web searching you'll find something.
I'm sure a printing company can get it for you, but they might have carton minimums that would cost a bundle.
Sorry I couldn't help more.