I would just make your own out of plywood or MDF. You can typically have a piece of wood cut for you at any lumber store. That way you can make it to the size you want. Then you can cover it with foil or a cake board cover like from Wilton.
I make my own boards all the time from scrap plywood or partical board or I have used two or three (depending upon the size and weight of the cake) pieces of heavy cardboard glued together. I always cover them with at least two thicknesses of aluminum foil.
Though I don't think that size is 'oversize' do be *sure!* it will go through any and all doors/openings it need to travel through. Don't get caught with a large cake not fitting through. I once had to take a large finished cake down a very narrow set of steps to a basement. Had to tip the board w/cake on it to do so.
what I do is piece together foam core boards with wide tape -- make two or three the size you need -- then set them cross ways so the seams are in different places and hot glue or tape the whole thing together -- it's light weight, easy and very very sturdy --
there maybe someone out there who thinks this will not work -- I understand that -- but I've been doing this for decades -- none of my cakes ever cracked or fell -- just plain works -- nobody has to try this -- but please don't dis me -- thanks
I tape the seams continually from side to side and across the top and the bottom -- then securely tape two or three of the boards together --
most everybody does everything different it's what makes us all unique individuals 
I actually like the foam core idea, -K8memphis, and having been an artist and commercial artist all of my life, I know it is very strong. If I sold cakes, it would be my choice for custom bases. Since it is just my hobby and I sell only the occasional cake, I just use the scrap lumber we have left over from construction projects.
Quote by @%username% on %date%
%body%