Reddit mentions of Poly-dowels Plastic White Dowel Rods for Tiered Cake Construction, 12 Inch X 1/4, Pack of 12

This example should help. Basically,
hollow plastic dowels and cake boards between each layer will support the weight of the tiers and keep them from collapsing. Depending on the diameter of each tier, you'll use anywhere from 3 to 7 dowels. Assemble a tier on top of a cake board with the same diameter. Looking down at the top of the tier, figure out where the dowels need to go. The dowels will go in about halfway towards the center of the cake in a triangle (3), square (4), cross (5) pattern, etc. Once you know where they need to go, cut the dowels to the height of that tier, then gently tap them all the way in. Do this with each tier to secure it. Then stack the tiers on top of each other. And if the tiers are particularly huge and heavy, you'll need a center dowel all the way through the cake (solid wood or metal). But this is only for massive cakes, usually. Voilá.

Edit: Fixed broken link