I just buy the window boxes from Michael’s or Hobby Lobby and pull off the plastic window. If the tiers are too wide, I use an xacto knife to cut it wider. The top two tiers stick out but I haven’t been able to find another option that isn’t expensive. These even fit:
http://cakesdecor.com/cakes/10902
http://cakesdecor.com/cakes/10909
http://cakesdecor.com/cakes/10905
