Any ideas where I can find boxes or plastic cake containers (preferable) that will accommodate tall cakes? All my cakes are 3 layers and everything I've found so far is just a little on the short side, especially if I put any kind of decoration on the top.

what I would do is make my own box or extend the regular box by securely propping open the top and covering the opening with plastic wrap — you can also get corrugated cardboard moving boxes to seal your cakes into if necessary — keeps cold cake climate controlled