Purchase it directly from the manufacturer PS.
SRS does not sell Active Belt from SRS website.
 
Must be purchased directly from the manufacturer.
https://www.pt-actuator.com/product/pt-active-belt-tensioner/
 
Feel the power of sustained G-forces like never before. TensionR™ adds an extra level of realism for the ultimate racing or flight experience.
 
It comes with a 5 point belt system and it's fully integrated with Sim Racing Studio software for an easy, plug-and-race/fly experience
 
Setup and troubleshooting guide can be found here.