ITEAD of Sonoff fame now sells a very inexpensive Zigbee USB dongle that should work out-of-the-box with both Zigbee2mqtt and the ZHA integration - Zigbee - Home Assistant Community
ITSTUFF CC2531 ZigBee USB Stick ioBroker openHAB FHEM SMA Antenna (Router firmware) : Amazon.co.uk: Electronics & Photo