I got a 8BITDO controller whenever I wanted to play classic games and arcade games on the Switch.
Specifically this one:
I think its compatible with both iOS and Android, along with Switch.
The trick is that you have to press a series of buttons in order to do bluetooth signal for specific devices, and a different series of buttons in order to do connect via bluetooth on Switch, and once you do that, I think you have to press the power button on a certain of times so that it knows to power on as a Switch controller, or as an Android controller.