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Your best bet is to go see a bootfitter at a reputable ski shop. Of course buying online is easier and cheaper, but the extra time and money is worth it. First, the bootfitter can change the boot shell to fit your specific footshape, and if they are good they will invite you to come back if there is a problem. Second they will often switch out the stock insole and set you up with an aftermarket insole trimmed to fit at a discount (check out your stock insole you will see that it offers little support). Lastly they can even make you a custom footbed that you can even use in future boots.
Build a relationship with your local ski shop because they can help you find a comfortable fit, even after a couple ski days.
Think about it. What the point of spending hard cash on a fancy boot without guarrantees of a proper fit?