The film in question isn't what really makes a backlight - that is the light pipe that has a pattern etched into the backside, to changed the angle of internal reflection so light can 'escape' through the face of the panel. Often these are a series of dots (moulded in) or bumps (moulded protrusions) but can also be V-grooves where the lines get progressively closer the further from the lightsource it is, to give an even brightness.
This creates a panel with lots of small light 'dots' or lines, the diffuser film then..well.. diffuses this to create an even light source across the panel. There is also a mirror/reflective film on the back because the lightpipe will kick out light from both flat sides, so at the back it needs to be reflected back into it. As LCD's (including TFT's of course) use a polariser, 50% of the light is wasted when a pixel is transparent (usually 'on' on TFT's, as they are negative displays). Some companies like 3M have created a polarising diffuser - this allows one polarisation to pass through, diffused, whilst reflecting the other polarising back, recycling the light. It's clever and can increase the brightness, or reduce the power consumption. I believe these diffusers are standard these days.
You could search for a PSP (play station portable) backlight replacement, as this is the whole assembly. Although I suspect this is slightly too large for that display. Rolling your own can be very tricky. Its easy to epoxy some LED's into the edge of acrylic, but to get an 'even' brightness across the panel requires very specific machining/moulding of the lightpipe. Should you wish to make your own, I highly recommend using a bright laptop screen for parts - it'll come with all the films you need, plus a big, thin, lightpipe to cut to size. I've made illuminated posters this way for less than £10.