[hooande]

This idea seems like it's 100% about distribution. If I owned an extreme sports rental shop, I would have an incentive to get more people to go out and participate in the activities. If the app was well made, reputable and secure I would consider putting up a sign in my store or whatnot.

People that run equipment rental stores probably have a facebook group or professional association. If you can befriend someone influential in one of those, it might be a good place to get started.