To get true industry insights, networking from outside the industry is often insufficient and can potentially lead you down a path of failure and infant mortality.
Yes, by networking you can get people to tell you things, but talk is cheap. People will tell you anything when they have no skin in the game, and you will be tempted to execute one someone's emotional rambles (or multiple persons' correlated rambles, which is even more convincing but no more correct) which they have not thought through all the way. If you're going to build a business, you need higher quality information than that.
Also without the right context in the target industry (or a similar industry), you won't really grasp the underlying nature of the inefficiencies (often a multifaceted quantity), which means there's a high probability you will falter in your execution. You're going be tempted to solve problems that people claim they have, but if you've ever worked in consulting, you will know that the key is not to solve the problem people SAY THEY HAVE, but the problem THEY ACTUALLY HAVE, which is often a different thing. This is consulting 101, and holds true for pretty much any industry.
In order to understand the real problem, you need to be in a position where you have to have had experienced the pain points somewhat first hand.
(The following applies to the more complex industries that I'm familiar with; the Christmas market industry is not as complex and not all of this may be applicable)
1) The most effective way, but not an easy way, is to work in your industry of interest for a year or two in a position that will expose you to data and people in the field. Typically this means data analyst or internal consultant. This way you will get an insider's view and gain street cred (do not underestimate how important this street cred is) if you should decide to build a product to sell to that industry.
[Too often I am approached by enterprise sales people who don't have an appreciation for my industry and try to sell me software to solve for problems they think I have but don't (e.g. anomaly detection? IoT? in my industry, we've been doing that for the past 3 decades under different guises).]
2) If you don't have that luxury, you can work for a consulting company that consults for the industry of interest. That way you will get a cross-organizational view of problems that are common in said industry. This also gains you some street cred, but not as much as (1).
3) The other option, also hard, is to either hire someone from the target industry or have someone who has a track record in said industry mentor you over a period of time. You need that repeated exposure to truly grasp the context. Elon Musk's companies, if I read his biography correctly, tend to take this approach.
If possible, make sure this person has skin in the game in your eventual organization, so this way their goals are aligned with yours. If these conditions apply, then you can apply first-principles thinking, but not before. Quality of information (experience) is paramount. I hate cliches, but be skeptical. Trust but verify.