Call your local Safeway - they may be able to help you out. I've bought two sheet cakes and a ton of cupcakes from my local Safeways (one in Pan Am shopping center, one in Arlington) and been happy with them. The cake and frosting are both good.
Worst case, they usually have a few pre-decorated sheet cakes on hand that are generic. Giant usually does too.
You could also call Amphora for something fancier. Their cakes are delicious.