You'll probably need to look online for it. I don't know of any craft stores that carry it. I will tell you though, they are quite pricey 300 dollars or so sometimes more.
Just do a search on "cake plateau" and eventually it will come up. $$$$
I bought an 18" square plateau very similar to this on ebay, it was about $250 a couple of years ago.
If you need this for one time only, try party rental places, some of the higher-end ones have some really nice pieces...you might get lucky!
Just for fun, I searched for this, and I cannot find it. I can find it round, and I've seen it square when I was looking for a different stand...
Closest I could find: http://www.amazon.com/dp/B000L4SEUI/?tag=cakecentral-20
another, not horrible: http://cgi.ebay.com/18-Square-Wedding-Cake-Stand-NEW-silver-Platter-Plate_W0QQitemZ250313052633QQcmdZViewItem?hash=item250313052633
Thank you so very much for your advice! AligotMatt, the one is very close to what I wanted! Thanks so much!
Have a great day all!
I found these sites when I was looking. Hope this helps!