Is it possible to buy (western quality) disposable nappies (diapers) in Costa Rica and/or Panama?
In Costa Rica we are planning to be in: La Fortuna, Monteverde, Montezuma, Manuel Antonio In Panama: Boquete, Bocas del Torro, Panama City
I can confirm after checking in person, that nappies are available pretty much everywhere in Panama and Costa Rica (at least in the locations indicated in the question). Obviously, don't count on finding your favourite brand though.
Sadly this information is not the only one that hasn't been updated since the first edition of the guide book years ago :(
Lonely Planet advises that you bring them (or stock up in San Jose).
I'm not sure about Panama but I would assume the same.