I purchase mine at www.countrykitchensa.com
Remember that what people are paying for is your talent and the time it takes to decorate. Most decorators charge between 50% of the regular cost to 100% of the regular cost for dummy cakes. IMHO, to pay for a decorated dummy cake and the sheet cakes really is going to come out the same or more expensive than just getting a real wedding cake to begin with.
For example, if you charge $500 for a wedding cake that will feed 100 ($5/serving), and you charge 50% of that for the dummy cake, then the dummy cake will be $250. If you charge $300 for sheet cake to feed 100 ($3/serving), then the total cost for the couple will be $550, which is $50 more than if they'd just gotten the wedding cake.