To the best of my knowledge, DVI and VGA are standard configurations and widely available. You could probably find what you need at any local electronics store and look at them in person before you buy.
At one time, Apple did have an ADC (Apple Display Connector) that is proprietory and found on things like a PowerMac G4 desktop. It was for an Apple Studio Display which came with this cable.
Does your PowerBook also have a DVI connector? If so, you may just need a DVI to DVI cable, which would be even easier to find. Alternatively, you might be able to use your short adaptor and connect a longer DVI cable to it.