It is very hard to find a keyboard that does not allow for a headphone. Even the cheapest ones that I have ever seen seem to have headphone jacks. Cheap or expensive, they all have headphone jacks.
It is very easy to find a keyboard that requires either headphones or an amp(does not have onboard speakers) Most professional keyboards do not have onboard speakers. There are notable exceptions in the arranger and digital piano groups. Most of your portable arrangers have onboard speakers.
berbie