I buy the cardboard boards at the party stores, but once the cakes on on them and decorated, they aren't as sturdy as they should be. I have had buttercream cakes "crack" across the middle when moving it I have started hot gluing a couple of boards together and that has helped a lot. When a client came today to pick up a cake (3 tiers, fondant) the bottom tier started "buckling". I think the bottom board was not supporting it enough. Should I be using plastic boards?
I was using cardboard and now i am actually just converting to masonite...they are much more expensive but so worth it...
I use either foam board or cake drums for up to 3-4 tiers, masonite only if it is super heavy. Be careful which brand of cake drums you buy, some brands are stronger than others - I order mine from DecoPac or CK Products.
I use 1/2" foamcore boards for up to 3 tiers and I double it for anything taller. I also use 3/16" foamcore boards under each tier.
I either use foam core (preferred) OR I have used two cake boards glued together. Also works great.
I use the latter when I don't feel too much like cutting the foam core.