The FEI leases that I’ve seen have been found through word of mouth, usually with trainers.
There are two GP horses in the barn where I take lessons. One is retired from upper level work and is a schoolmaster for someone who is moving up the levels and wants to learn the movements on their young horse. The schoolmaster is owner by another client in the barn, so the horse remains under the same care, so it’s much easier for everyone.
The other horse is a horse on free lease that my trainer got from a YR who is now off at college and doesn’t have time to ride. The horse is 24 so he’s not out showing at GP, but he can do all the movements, and is fantastic for learning the movements on. My trainer pays for shoes and the owner pays for the vet work. The board is covered easily by the fact my trainer has him at her barn. Several different people take lessons on him, including me. It’s an opportunity that would be hard to come by any other way, without a lot of luck or a lot of money.
That’s where I think others are right - you have to start with dressage lessons first, get connected with a trainer, and then the opportunities will be easier to come by.
It is worth the effort though if you are serious, OP. I went from doing 1st level work to doing the 2nd and 3rd level in not too long of a time period. When I first started though, I felt like a crap rider. I had been successful in other disciplines, but I’d get passage instead of trot, ones instead of working canter, and riding this particular schoolmaster felt like an outrageous workout. I was overjoyed on the rare occasions my trainer had to cancel for one reason or another. Then after a few months, I became more solid in my position and now I find the power quite fun to experience. In the beginning though it was difficult. Like taking a drink from a firehose.