That's because they don't exist.
Nobody pays for Beta testers, those happen out in the wild.
What you are looking for is a Tester job as a title, but that involves actually debugging and understanding code not just playing with stuff and providing feedback.
There's a **** load of testing done, long before it comes close to the market and those are real jobs. There really is very little need for anything beyond that and especially not to pay someone for it. Beta testing is more of a carrot for early adopters to feel cool about themselves these days.
On the odd occasion you might find some once off market research done for a given product/app which is marginally paid for.