Hate to say it, but by you or someone else making it. Searched high and low for one for our son and ended up making it ourselves.
Buy a plain iced cake from Tesco/asda etc, orange, blue and black food colouring and icing sugar, mix colouring with icing sugar and little water, paint Nemo on (may have to rename 2 year old or change name on cake) 1 hour tops to do
Plain cake and get some cake toppers from amazon http://www.amazon.co.uk/dp/B00H162GB2/?tag=ducatiforum-21 he can play with the toys afterwards as well. Or if he wants to eat them some of these 12 Large Stand Up Edible Premium Wafer Paper Finding Nemo / Dory Cupcake Toppers - by Carlton Trading: Amazon.co.uk: Kitchen & Home
eBay has Nemo cake toppers, you can choose the name, and they'll print one off (hopefully using edible ink)