My husband got me a piece of osb board or something like that at Lowes. I think it was like 24 X24 and it was only $3.oo. If you had somebody to cut it down to size that would be even better. I like it because it was nice and stable and I can use it again. Or you could cut a strog piece of card board from a box or something like that. I know lots of people use different things. Here are just a couple ideas.
I have done two things for larger cakeboards:
1) go to walmart and buy one of those three legged round side tables that are plain jane (they are like $6) and use that. I believe i measured it out to be 19" round.
2) go to your local home improvement store and get some MDF or plywood. They will make a few cuts for you for free to whatever size. I have a table saw at home so i cut my own when i need it.
HTH!
I've used cardboard for very large cakes before. I usually go to Lowe's and get some discarded refrigerator boxes. I believe I make it about 1" thick and have the corrugated lines run perpendicular to each other for each layer of cardboard. That way I don't have to worry about getting the board back. I have plywood bases also, but it seems like it takes at least 2 weeks to get them back!
Thans for all these suggestions. I totaly forgot about foamboard, but as susggested would at least double it up. The cake is only going to be the 16" square (torted) and a 1-layer heart on top so it should not be as heavy. I also like the idea of using either plywood or other wood. Being it is my daughter's shower, won't have to worry about getting it back.
I suggest using plywood... I used some REALLY heavy duty cardboard that my husband got at work and when I went to move the cake after smoothing it all out, I got a ton of cracks and had to redo it all... then we put plywood underneath that and it worked out great. i weighed my 16x16 cake and it was 20 lbs.!
Oriented strand board (OSB) is a type of wodden "mat" that is made from wooden sheets that have been pressed together in alternating layers. It is a mat of wood, wax, and a waterproof binder. It is used to frame houses and roofs!! it's lightweight and comes as a panel. you can cut it and use it as a base board for cakes. It's kind of like plywood (sawdust and glue), but is better b/c it isn't as heavy. You find it in the home improvement store with plywood and other manly building materials 
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