My best guess would be a large department / food store like Real or Globus.

Now for the bad part (if you live in the USA). If US Customs finds them on you, they will be seized at the border. Kinder Eggs are not allowed in the US (the enclosed toy is considered to be a hazard to children). The ironic thing is, you could probably open up the Eggs and remove all the toys, put them in a plastic baggie and sail through customs with your declared 'toys', but if they are still enclosed in the Kinder Egg, it is illegal.