I often have the same problem! My cakes are a minimum of 5" tall and I usually have customers picking up tiered cakes too. The regular cake boxes just aren't tall enough, grrr!
Go to your local Business Depot or Staples (Walmart, Kmart, whatever!) and get Banker's file boxes. (I get 6-packs for about $13 dollars here in Ontario.)
They are VERY strong, sturdy and about 11" tall. They have built-in handles and are a good 12" wide usually. They work perfectly with a tall cake or even tiered ones.
What I do is cut out a square of non-slip- rubber matting for the bottom of the box, then lay the cake on its board on top. SOO perfect for transporting cakes!