The Trinket m0 costs $9 and can be found anywhere on online shop and obviously directly from the manufacturer Adafruit.com
Trinket m0 Device
The Trinket m0 3,3V device is very popular and easy to find in your area or in online shop like Mouser, Amazon, Gotronic, ThePi Hut, Robotshop, Lextronic, …
The Teensy LC from PJRC is no longer available unfortunately