Fellers has 3m Scothcal 50, clear, 48". I use it's white exclusively for my 1204.
Backlite has two situations just discussed in detail in Sign Business magazine last issue.
1. lighted 24/7. use clear vinyl with "back light profile" The print will look darker when not lit, but look right when lights up.
2. Lighted nightly only: use normal profile print on clear vinyl once, then print on translucent white again, apply clear to lexan or acrylic, then apply the translucent white over with perfect registration. This way it will look right when light is off or on.