What shoes should I buy my bridesmaids for our December reception?

I'm getting married December 1st. I told my bridesmaids they could buy whatever shoes they wanted for the ceremony, as long as they're solid black (the dresses are black with a silver sash). What kind of shoes/flip flops/slippers should I buy them to wear at the reception? Everyone always gets flip flops but I'm worried about how cold it will be (in CT). I was thinking maybe the isotoner ballerina slippers, but they're so thin. I really want them to be comfortable, does anyone have any suggestions and website links? Thanks!!