Momentary switch? ... that's really rare these days.
The common types of switches found on flashlights today:
(1) clicky switch - this type of switch interrupts main power when in the off position. Think of it like a light switch on your wall. No current flows when it is off. Switches of this type have a tiny rotating piece inside that works like a pushbutton ballpoint pen so that successive clicks open and close the connection.
(2) Electronic switch - this is a simple momentary pushbutton combined with a control circuit. The control circuit is usually part of the driver programming. Main current to the driver is on so long as the light is assembled with a battery inside. Main current doesn't flow directly through the switch so the switch and switch wires can be very lightweight. The switch acts as an input to tell the control circuit that it has been depressed so the control circuit can take action in line with its programming. It's like pressing a key on your computer keyboard. Doing so is merely the input that tells your computer to do something.
(3) Twisty switch - very simple. Functionally, just like a clicky switch. When the light is unscrewed, the circuit to the battery is interrupted.
When you say you want a "momentary" switch on your flashlight, it sounds to me like you want a light that only works while the button is depressed. Essentially a momentary pushbutton through which main power flows (not part of an electronic switch). Very few manufacturers make lights with that particular switch mechanism. There are some lights with electronic switches that can enter a momentary mode that mimics that type of user interface however.