Pretty much any nrf52840 that you can load an arduino sketch onto. E.g.
SEEED XIA0 nrf52840. Super small - will take a lipo if you solder onto the back batt pins
Adafruit nrf52840 express. A great develeopment board. Has a spare button for you to use as either a pairing button or sending button.
Adafruit itsybitsy nrf52840. Same as the feather - but smaller and slight less flashy
And for the dongle
This is nice as its all in one piece. nrf52840 USB key