datasheet: https://www.sensirion.com/en/environmental-sensors/humidity-sensors/humidity-temperature-sensor-sht2x-digital-i2c-accurate/

lib https://github.com/SodaqMoja/Sodaq_SHT2x

SHT21 Module: GY-213V-SHT21 (5$)

SHT20: 

AliExpress: 3.69$

SHT25: self made from SHT20 module (4$) and SHT25 sensor (10$)

AliExpress: 3.5$ for 2