I can easily find boards for 12 by 18 cake pan but what box  do you use,? I can't find any that goes with that particular size. Thanks in advance 

Many box makers make custom boxes (you may need to buy a few). Just type the size into google.

I always put my 12x18 cakes on standard 1/2 sheet boards therefor they fit in the 1/2 sheet box.  It is a tight fit -  very little room for bottom borders but much easier than trying to find special boards and boxes.  HTH

I was thinking the same thing  too, the border would be tight 

thanks for all the help 

Wilton makes a 14x19 box that they say can be used for a 12x18 cake. Here's their website.

http://www.wilton.com/14-x-19-plain-cake-box/415-945.html#sz=30&start=43