It is tough, very tough, even now most of my well paying clients are repeat clients; I do a ton of one hit wonders like headshots, photo studio work, etc. but repeat business and word of mouth is the real secret. Of course that is the photography side of things. For video many of my customers started with photography and eventually needed video then kept coming back to me for both.
My particular video niche is big events (boat races, car shows, food festivals, fashion shows, etc). That niche in turn spawns tons of side projects (photo shoots, promo videos, talking heads, training videos, etc. etc). So if I were starting over right now today I would just Google events in my area and start going to them and filming them for free.
I have a branded shirt with my business name on it, I also have business cards, and I would wear my shirt and pass my cards out literally everywhere. I would shoot entire highlight reels of the events, really focus on handing out my business cards to the vendors and try to figure out who the organizers were. If I could work my way up to the organizers and main sponsors I would offer to tag them on IG and FB when the video and photos were out and give them my business cards which would ensure they saw my work.
I would give it less than 3 months of that before I started having sponsors, organizers, and event attendees reaching out to me for photos and video. Within about 6 months in a reasonably large city I would have my whole network built back up. I keep stressing photography because in my niche it is so important. So many people contact me for photography and they never even considered video until I recommend it. One thing leads to another and next thing you know I've been working with that client for years. Events are also great because they are recurring year after year so that is repeat business if you work for the main sponsor or the event organizers.
Almost all of my business comes from these events, people at these events are getting married (they need wedding videos), they are having kids (birthday party videos / gender reveals, etc.), they run their own business (business promo videos, product demos, etc needed), etc. etc., I shoot a ton of small stuff thanks to these events. To this day if I haven't handed out 500 cards at an event that I am filming then I am not doing it right. I never let up on the networking because events, sponsors, and attendees come and go each year so relying too heavily on any one of them for revenue the next year is a recipe for failure if they don't host or sponsor that event the next year.
Of course my approach only works in a large city or somewhere where you can easily drive to a large city. Where I live I could literally shoot nearly any type of event I wanted on any given weekend (sports, food festivals, music festivals, races, fashion shows, etc.). I once shot a live concert for a customer that needed a city activities promo video and ended up shooting a music video for the band because I gave them my card after the concert.
I have also had plenty of offers to work directly for the sponsors on their media team but I like being freelance, after working literally everything for the past 10yrs I am not interested in joining a team; but if that is your thing that could be the way into a more predictable career field.