Actually the best in my opinion is "backlit film", this can be done at Kinkos or most printers. Backlit film is exactly what you are using it for, to have a light behind a picture to light it up. All those movie posters, etc., that you see at the malls and stuff, where it is a poster lit up from the back - those are printed on backlit film usually.
You can buy this stuff to print on a home inkjet printer, but I think you can only get 8.5x11 sheets so you would end up with seams. I would recomend a pinter like Kinkos if you can afford it.
A standard flourescent light (like described above) is fine.