This year I decided to get a set of sno tires fot the car - over the last year I have been also acquiring (4) stock prius wheels when I could find one for a good value. Recently I ran into a deal for a set of (4) 195/60/r15 Blizzaks WS70's with minimal wear, so I snapped them up. Took the whole lot to the local tire place and they had them mounted and balanced in about 15 minutes. If you do decide to go used on the tires look for either 185 65 r15 the stock size or you could go up one size as I did to 195 60 r15. You will lose some MPG's with the larger tires size, but your handling will increase.
If you change your own tires, invest in a good 3ton jack. I always lift the car a bit to get the weight off but leave the tires touching the ground, that way when i loosen the lugs the wheel dont spin. Then i raise the car so the wheels don't touch the ground by an inch and remove the lugs the rest of the way. When tightening, i screw on with hand to start, impact hammer just before snug. lower car till wheel touch ground barely, then finish with torque wrench in a star pattern to 80 ft lbs. Not using a torque wrench can leave you with undesirable results. See: Wheel on side of road, or impossible to remove on side of road after you over tightened with an impact wrench.
I will update you when we can test out the grip in some bad weather, but definitley not fearing the snow anymore.