Don’t want to be a “Debbie Downer”, but finding a trainer with an upper level schoolmaster is possibly harder then riding a GP test. The few trainers I know that have school horses are lower level horses - Tr/1st, maybe 2nd level. If you want to work towards riding higher quality horses, the better route is to find someone that will take you on as a working student - and realize it could be a few years before you are on any of the higher trained horses, and you’ll spend a lot of time working on non-riding stuff - grooming, mucking, etc. Having said that - the more horses you can ride, the better your long term education will be.
Many International Dressage riders started in their adult life - some came over from jumpers or eventing.
There are a lot of good books out there - but books are just the start - you really need to see and feel real horses to get a true understanding of biomechanics. Just as you need to actually ride to learn how to ride - books are just a small part of the formula. Having said that, Mary Wanless is a good start for biomechanics - a little quirky, but a good place to start reading. And look for clinics, symposiums, etc - people like Heather Blitz are wonderful for biomechanics info.