MQTT device for sensors

I’m trying to connect some sensors to my Home Assistant installation.

Ideally, I’d like to buy just cheap sensors and connect them to a smart device.

A simple setup could be something like a proximity sensor connected to a sonoff mini or a shelly device and then have that send the sensors status to HA.

However, both the sonoff mini and shelly include a relay to switch something on or off. There is no real need for that, the only thing I want is for it to send an MQTT message to home assistant when the sensor changes.

The other route would be to use an ESP, but in that case, I’d have to buy a separate power supply.

Is there some middle ground where you basically have something like the sonoff or shelly, but without the relay?

Or, alternatively, an ESP with a power input off mains that can be used for this?