My sister just got some really cute ones at Kmart! I'd also check TJMaxx and Target. Check in both women's and men's department. Sometimes the men's dress socks are tall enough to work!
I just pinned this the other day- I think it's genius! Using old sweater sleeves as boot socks. You can find old sweaters at thrift stores too. This could either remove bulk or let you wear whatever cozy socks you want if you need the extra warmth.
Eta- this obviously won't work as a gift for your SIL, but I thought it might be useful to some of you