We (mostly me and @aufdenkampe, I think) have been purchasing them directly from Yosemitech because they are so darn cost-effective without compromising quality. Currently they are subject to the new 25% “Trump Tariff,” but they are still proving to be cost effective. Note that to use these sensors, the Mayfly needs a “wing” to boost power and translate modbus communication. The most current documentation is here: https://github.com/EnviroDIY/SensorModbusMaster/tree/master/hardware/Modbus-Mayfly_WingShield (with a little extra background and links here: https://github.com/EnviroDIY/SensorModbusMaster/tree/master/hardware).
Yosemitech is a small company and has been good to work with. They make sensors when you order/pay, so the turn-around time, with shipping and the inevitable hang up in US customs, takes ~2 months to receive sensors. We are trying to work with them on how they declare and describe the sensors to smooth the ride through customs. The purchasing process is a little tricky because they don’t take credit cards (we usually wire transfer). They also have a warranty/maintenance situation that we basically cannot maintain by purchasing them for use in the US (last I checked).
The process is involved, so I’m all ears for alternatives, and will be watching this thread.