My son also wanted an airplane cake, which was easier than his Toy Story request the year before!!
You have 2 options:
1. Use the base of a plane model and put cake on top and around it, but for that you need a very flat, large plane. See the link below for an example.
http://www.instructables.com/id/Aiplane/
2. Alternatively you can make your own from scratch. See below.
http://www.parenting.com/article/how-to-make-an-airplane-birthday-cake
Good luck
There are lots of sites that let your order cakes and specify exactly what kind of cake and icing you want. Also you can get a message added on.
Take a look at these couple of sites to give you some ideas;
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