Does anyone know where to purchase larger size cake pedestals... 14 inches or larger in diameter? I see so many pretty pictures of cakes on pedestals, but can only find smaller ones.

If you are lucky, you can find ones a Marshalls, TJ Maxx or Ross. Also check yout Antique stores because they do sell some.

do you mean the pedistal stands or the cake plateaus?

either way, try eBay.
I have a bunch from random places.
I just bought a 14" cake plateau (very slightly used) and a cupcake stand of eBay for a wedding tomorrow.