The worst problem to have in a ski boot is a boot that is too big. There is no great fix- boot fitters expand boots, not shrink them. The proper insole will make your foot shorter, actually creating MORE room in the boot. Your best bet is to get a good footbed (it will at least hold your foot in place), have a boot-fitter put some volume shims under your liner, and then he should pad your liner, around the ankle and heel area.
This will probably at least run you about $100, depending on what kind of footbeds you get. And like I said, it wont fix the problem. The only real solution is to get new boots.