dealextreme.com have a ridiculously huge range of them, though it can take a while for stuff to arrive.

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Second on the DX suggestion, they rock for prices on just about anything you can have legally shipped to the US. I just got an original TiBook 500 myself, and I thought it'd be nice to do something like this maybe, sometime in the future when I'm more daring. Since of the black semi-seethrough keyboard, it'd be pretty easy to light these suckers up. and the white plasticky-looking sticker on the bottom of the keyboard looks like it'd be able to transfer off some sort of a glow if light was applied to it. I'd suggest looking into seeing about that, as well as where you'd put a switch. For me, it'd be in the space that would have otherwise housed the modem in mine. I took it out because I'm never going to use it... so now I have space to put something in the area next to the GPU as far as that goes. You can even see if there's some way to tap into the modem connector's power connection right there... that is, if you don't use your modem either.