I got an odd wedding cake to do. Knowing I would have questions about this, I tried to steer the bride to a more traditional cake. Not a chance. I know the cake will be beautiful when completed; it is very ellegantly designed. The tiers are in this order-8 square, 6 round single layer,12 square, 10 round single, 14 square. I think it will stand just fine. My question is what do I use under the 8 and 12 inch cakes? They are sitting on smaller cakes. And where can I get a cake board that will hold a 5 tier wedding cake?

It's not the cake board that holds up the 5 tiers - it's your support system. The SPS will work - can't think of the name of the other system, but it's expensive
There are several 'upside down' cakes posted that might help you. Look thru the galleries and contact the creator for some help.

First of all, I would use styrofoam cake "dummies" for the 6 and 10 inch rounds, not actual cake. This will make the overall construction much sturdier and easier. Instead of cardboard rounds, I would use plastic cake plates under the 8 and 12 inch squares. For the base of the cake, I would use a piece of plywood 1/2" thick covered in cake foil.

HTH! Can't wait to see pics of the finished cake!