Make one yourself, my son decided that this year he was too old to have a theme cake so asked for homemade chocolate cake instead, his friends all said it was way better than the shop bought one.
You can get some great icing cake toppers from Ebay where they will print any picture/text you like on them & then you just freeze them and then quickly stick it on top of a plain cake. Making your ownis always cheaper (& much nicer!) than buying one - depends how many it needs to feed & whether you have a big enough tin ... you can hire these from party shops / cake shops though.
On my sons 13th birthday recently we got a dozen Krispy Creme (chocolate dreamcakes) donuts as this is his current favourite cake. They went down very well at the party!
The chocolate birthday cake in Tesco is yummy - it has chocolate balloons on the top but they would be easy to take off. The nicest choccy cake IMO and Im actually not keen on chocolate cake!