I am making a tiered cake, and bottom tier is going to be 14" so the cake board I am making it 18". Where do you purchase a 18x18x? box to transport a tiered cake?

Staples, Office Depot, Office Max, etc.

https://www.bakerycrafts.com/ItemSearchResults.aspx?SrchStr=DS-4A

I bought an 18 x 18 x 24 inch box at Wal-Mart to transport my Topsy Turvy cake in. It was just a few dollars if I remember correctly. I believe it is a box used for moving.

U-haul, UPS store, Home depot