I am currently finishing up a music-related documentary series and need a bit of advice regarding finding reputable distributors. Is at always necessary to go that route rather than directly through to retailers?
Depends what you mean by "retailers." I know some folks who make a grand living by making documentaries for state parks and only selling through the park's gift shops, local hotels, and the like.
If you mean retailers like Barnes & Noble rather than a local hotel, then yeah, distribution is your only shot. The purchases are done at a corporate level, and while they absolutely make smaller buys (such as from small presses or for specific regions), they can buy from boatloads of them at once with a single account through distributors like Ingram, Publishers Group West, etc. It's a matter of efficiency.
I was in this end of retailing (as it happens, at Barnes & Noble) for a few years, and never heard about disreputable distributors. But you'll find that many of them, like publishers, are minutely specialized -- regional, scholarly, religious, etc. This more than anything else will dictate which way you might start looking.
But if you are indeed looking to cover an area that you can reach by car, or few enough units that you're willing to handle order fullfillment and returns by yourself, then go for it.
As with all such things, then, the question is, what's your business model? What are you trying to do?