... The power supply I found (I linked to it above) says it has 9W... this won't be a problem will it?
No problem! 6V x 1.5Amps = 9W. Perfect!
Sorry about the confusion. I know that was a lot of information... I was trying to say, that a power supply could be marked 9W for the input (power from the wall), but you might not be able to get the get 9W out of it (into your motor). The regulatory agencies usually require anything that's plugged into the wall to be marked with the maximum power it takes from the wall-outlet.
If it says 6V, 1.5A, that's the output, and that's what you want to know. They will almost always give you the output voltage & current, so you can can always calculate the output power (Watts) if you need to.
If the specs say "9W power supply", that's the output too (and you can always check the calculations). But, the markings on it could have a higher number that indicates the wall-power.