Try thoughttechnology.com.

Strain gauge output needs software to process it. They can advise you - I use their Biograph Infiniti hardware and software for HRV with good results.

Christopher Gilbert Thank you!

Hi. Keep in mind that some devices will give you a derivative of the respiratory movement like :

https://www.adinstruments.com/products/respiratory-belt-transducer

this is good for respiratory rate but not if you are also interested in respiratory amplitude (kind of respiratory depth measure)

I have used the following system :

V94-19 ANEROID CHEST BELLOWS +

V94-05 DIFFERENTIAL ANEROID PRESSURE TRANSDUCER +

V72-25B RERSISTIVE BRIDGE STRAIN-GAGE TRANSDUCER COUPLER

this system is very old but works perfectly to monitor breathing movement.

I compared it with respiratory air flow measurement. In most cases the respiratory depth (volume) is highly correlated with tidal volume measured by pneumotachometer.

Good luck

I recommend the device of Russian production "Spiroarteriokardiortmograf".