Old Navy: Adoraboots for $4.99 (originally $39.50)
Old Navy has a great discount on these Adoraboots, originally $39.50, now only $4.99. They have sizes 9 and 10 left in stock, in the colour concrete. Rumour has it that they fit small from everything I have read, so you might want to order a size up from your usual.
Not sure how warm they would be, but for $5 they would be an awesome boot back-up. In stores they are not $5 yet, so far this is just online. Free shipping is at $50 so you might want to check out other sale items at Old Navy, The Gap or Banana Republic to stock up.