Philips FL8D was not available to me, locally. However, any 12" T5 8-watt light bulb will work.
I tried the following alternatives:
Philips FLT5/SOFTWHITE - Terrible replacement. A very noticeable yellow-tinge (as opposed to the original bright white).
Philips FLT5/CW (Cool White) - Better, but not perfect. A slight tinge of yellow.