You need backlight media. It is a transparency media specifically designed for backlighting. Some of it is front print, some of it is back print. Obviously for back print, you need to reverse your image.
Kinkos may not have backlight media. You may need to go to a real service bureau to get it. When you talk to people about printing, ask them some questions about file format and resolution and color matching. If they don't seem to know what they're talking about, your results might not be what you expect.
Bob