Thanks to all who made recommendations.
I tried calling the Big Carrot for a vegan chocolate cake, but they couldn't get one in time for the celebrations. Fresh was selling a vegan chocolate cake for $55!!! Called Vegatarian Haven on Baldwin, and they were selling the same cake for $37. Can you believe the almost $20 difference in price??? Insane.
Needless to say, I've always loved Vegatarian Haven's vegan chocolate cake so I was thrilled to know that they sold them whole as well. No one at the party clued in that it was vegan until I mentioned it. It was delicious - highly recommended for your vegan party needs.