Short leafswitch buttons, or screw-type Sanwa/Seimitsus, will probably be the best bets.
Momentary means that the switch makes connection only while the button is held, and then disconnects when the button is released.
The other type of button would be an On/Off button, that toggles each time it is hit.
With momentary buttons, you will see two types.
The first is Normally Open (NO), which is the most common, and is used in arcade applications.
These buttons are "normally" open (disconnected) when released, and closed (connected) when pressed.
The second type is Normally Closed (NC).
When they are not pressed, they make contact, and allow functions to occur.
When they are pressed, they break contact, and stop whatever process was occurring.