A true cake shop will have square boards, as do many online cake supply places. If you're in a pinch, you can always get the sheet cake boards (rectangles) and cut them down into squares. A Michael's or Hobby Lobby will usually have the rectangle ones.
You should be able to nose around on here and find some posts that mention websites for boxes and boards for future reference.
Good luck!
I use them all the time. I either cut down a round or rectangle one from Michael's or JoAnn's or go to the cake supply store and buy the square ones.
Debbie
Hobby Lobby carries a foam core board that is very inexpensive and very sturdy. You will love them. They are almost 1/2 thick and I cut them down all of the time and wouldn't use anything else. As far as the edges I use double sided tape and fasten ribbon around the edges for an additional element. Don't forget to cover it with the clear contact paper.
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