I found a very cute 18" doll, only $20 at Target. The brand is "BFC, Ink" and I think they're the cutest imitation-American Girl dolls I've seen. They're hinged and flexible like Barbies, but close in size to American Girl dolls.
Isn't she cute? To make her cardigan without having to actually knit, I purchased gray socks and made a simple sweater shape. Her blouse is a length of white blanket binding that I had on hand. I used a length of yellow ribbon for the tie--I wrapped 1/8" scarlet colored ribbon (the 50-cents-a roll-kind) around it to make the stripes, pressed it flat, and sewed the whole thing down in a tie shape.
Here's what it looks like from the inside.
It's a two-fer! A faux shirt/sweater combo, which means it's super easy for the kids to dress and undress the doll.
Then I made her a black robe lined with with red satin, and a quicky crocheted Gryffindor scarf.
Sorry her hand is turned all at a wonky angle. Must be some sorta freaky wizard trick.