A quick answer, based on what you are asking and how you asked it, perhaps zappers are not for you.
Outside sources, as you say, generally spring into action upon being funded by somebody else to investigate any particular situation. In other words, these kinds of studies are not generally done for free. Universities and other branches of the corporate federal government complex/establishment are among those who often report having done "ouside studies", but anyone who thinks these are done free of charge is in serious need of some loving help. Look into the infamous Steve Barrett AKA Quackbuster, his type of work may satisfy your particular view of what constitutes proof in this matter.
The long answer - try to keep in mind that Zappers generally exist within the realm of alternative health. "outside source" sounds nice but is also a term associated with other red-flag indicators of establishment convention where it is not unusual to find inside sources posing as outside sources promoting information as as proof; like the infamous independent studies that upon critical review the results of which are often found to be not all that independent but actually dependent upon who funded them, and why?.
Proof is a dicey subject, there is seldom legitimate consensus or agreement on what actually constitutes proof to begin with, for zappers or pretty much anything under the sun. Especially in the present technology-driven info era, proof has become pretty easy to fabricate and there is a major industry known as mainstream media with lots of corporate sponsors helping to keep their idea of proof propped up; an evening infront of the sparkle box, starting at 6PM with the barage of information/proof pandered as "nightly news" bares this out. People in general seem to be okay with accepting (alleged)proof being provided to them by somebody else, like an outside source. To qualify, another covnention of our culture is, people remaining silent or otherwise absent on such issues is usually taken (by the establishment as well as at times by the culture at large) as their implication of being okay with this IE> "tacit approval", another establishment-wraught buzzword. Within the realm of alternative health it is not uncommon to find people maintaining the principal of accumulating facts on their own through their own research and experiementations IE> due diligence. It's your choice. If you are content to rely on somebody else providing information purported as facts, go for it. It is worth pointing this out since it may impact and possibly hinder your own particular zapper fact-finding methods.
I agree $189 is a relatively decent chunk of money to spend on something that may or may not do what it claims to do..... sigh, too bad people in general are not as vigilant with this same kind principle when it comes to some of the more routine and engrained ways that they are separated from their principal - money. Next time you see a paycheck, just count the number of separate deductions, each in it's own way money you are spending, willingly or not, for something that may not do what it is claimed to do, a big collection of somethings that I promise is not easy for you to prove one way or the other.