Looks like the ones used in the study are an order of magnitude more expensive than the ones on amazon.

Amazon (Plus model), $70: https://www.amazon.com/POWERbreathe-Fitness-Breathing-Traine...

From manufacturer (K3 model), $500: https://www.powerbreathe.com/product/k3/

There are various inhalation resistance trainers on Amazon for as little as $20. I'm doubtful the more expensive devices offer any benefits over the less expensive models. In fact the simpler less expensive models look much easier to take apart and clean. As long as you are breathing against resistance, you're getting a benefit.