You can also make/cut your own insole, out of something like foam. It should be wide enough to wrap around the sides of your toes. If how isn't obvious, look at my web page (see page on bottom of this message) on modifying boot fit.
You might even be able to attach athletic tape to the bottom of your current insole in such a way as to create "wings" on the side of the insole that take up the space on the sides of your toes.
BTW, if only your toes were loose, you probably wouldn't slide around. Chances are, it is loose a little back from there as well.