If you only want them for receiving calls, you should be able to pick one up in most supermarkets.

Aldi as being one of the big ones has good rates (and recently started having ,24 euro per mb for data traffic), as have the others you can pick up.

should be roughly 15 euros or such with some value on it.

as receiving of calls and sms is free it does not really matter which one you choose unless all of your partners are on a special provider where it would make sense to stay in the network.

hth