Working on the premise that this is an issue, then I would say it's practically impossible to avoid a certain degree of railroading when using a prepared adventure. I mean, you have to give your guys/gals a gentle nudge towards what is there, right? You have to at least show them where the adventure is and try to pull them in. And then, sure, you can leave it up to them to either take the bait or not.

It's also possible to write an adventure that railroads as little as possible. This would basically come down to working out locations and perhaps a sequence of events that your players can influence or--if they choose not to participate--that culminates in a certain event that may or may not influence them. And that adventure may end up being as vague as my previous sentence, but at least it will not do too much to railroad.

So how to get GMs that can host a non-railroady adventure?

I would say that GMs that railroad do so because they want to: they are focused on their idea for the game/story. After all, GMs are no blank slates that sacrifice themselves so that others can have fun; they have expectations of their own and play the game for their own entertainment. Oftentimes (and especially with new GMs) these expectations are in the form of knowing what story they want to play. That doesn't mean they 'lack inspirational material;' it means they have a certain goal for their own entertainment.

To solve this, you'd need GMs who focus on a different part of the role-playing experience and are perhaps more interested in group dynamics, applications of rules, detailed world-building, and so on. Unfortunately for you, most GMs get into the game because they enjoy playing out the story they have in their heads. I don't think you should dismiss these people by saying 'go write a novel' as much as you should dismiss people who don't want to be railroaded by saying 'go play in a sandbox.' Over time, these guys/gals can grow to like the hobby for other reasons than just projecting their story and may become more flexible.

But it's not all GMs, though. A non-railroady adventure requires players that can deal with that freedom to be fun. And to be fair, a lot of players may complain about railroading, but are often way too laid back in the game to be able to make decisions without some sense of direction, a few gentle nudges, or even being outright told what to do.

I am fairly sure that there is a way to try to teach GMs the skills necessary to improvise and avoid railroading, but the conclusion of my story is that a lot of GMs simply do not want to (yet) because they find it less entertaining