Weddingbee boards

If you're crafty, make your own!  http://www.intimateweddings.com/blog/vintage-cake-stands-make-your-own/

Or do you want the cake stands to all be the same style, but at different heights?  In which case, you can buy four of the same cake stand, and put a book or two under the tablecloth where you want the cakes to sit.  Then drape the tablecloth over them.

I made my own using glass plates and glass candle sticks - similar to the link above.  You can use anything for the base, thicker pillar chandle holders, drinking glasses, etc. 

gcwest's suggestions for stacking books beneath some of them is good as well!