I trailered to competitions with my sometimes conditioning partner for over a year before buying my own little rig (which was liberating - you can only ask for favors for so long…) When she couldn’t take us to a specific ride once, I found someone on “ridecamp” forum (endurance.net) to take me instead. You can try that venue as well.
It’s all about reaching out and expanding your network. Once you’ve been to your first ride, you’ll be hooked - and hooked up with willing new friends You may also want to volunteer first and go without your horse, just to learn and check it out.
One general rule of thumb is that you may have to ride with the person that takes you. Depending on the horses’ personality and experience, they may really buddy up on the trailer ride and being tied to the trailer together the night before. Some horses get very upset when left behind in camp, or at the start/during the actual ride, which can negatively affect their recoveries and how they behave at the vet checks (impatience, calling for their buddy, high pulse.) That’s another reason why I love traveling on my own. But occasionally I still trailer-pool with someone and after six seasons, my horse doesn’t really worry about much of anything anymore.
Have fun exploring the sport and good luck!