Using a switch to select the voltage will deplete the lowest bank batteries faster than the upper bank batteries. More eficient design is using a pulse width modulation (PWM) and 555 IC, as someone suggested. Using a switch and three or four different resistors will achieve the same heat effect, and be more battery efficient.
There are plenty of designs using a PWM and 555IC. You will need a MOSFET to provide the high current, but those are inexpensive. The insoles have around ~10 Ohm resistance, so roughly 0.75A will flow if you use Li_ion 7.6V batteries, multiplied by the duty cycle of the pulse.
Commercial boot heaters all use PWM. I own the therm-ic basic and use 4 2300 mAh NiMh batteries (4.8V, 20$ for pack of 8), which is more than enough for a day of skiing on setting of 1 or 2. I never needed the highest setting. Batteries will last few seasons (the cold is their enemy). The ability to replace the batteries was the reason I close the Therm-Ic basic.
In my opinion, the best route is to buy the insoles (~ 40$/pair for those compatible with Therm-ic), and build the controller yourself. Either NiMh or Li-Ion batteries will work, although Li-Ion will keep you warmer for the same capacity battery due to the higher voltage (7.6 V vs. 4.8 V).
The newer versions of the Therm-ic and Hotronic have a controller IC and control the level by up-dows buttons (better than the sliders, as they sometimes move by accident). I don't know what IC they use, if you know, please let me know.
BTW, I like the use of USB connector by BigMahe.