It's almost always bet to buy the wheels and tires together from TireRack or DiscountTire and have them shipped as a unit. It saves on shipping and mounting/balancing. If you are not in a hurry you can wait until DiscountTire.com runs their $100 rebate on 4 tires or 4 wheels and possibly double up!
The Prius c has a different bolt pattern but there are some nice wheels available for it. Go to TireRack.com and check out the Kosei K4R and the Motegi SP10. You want the lightest wheel you can afford. Wheels for 2012 Toyota Prius c Two