Have you bought the breeding barn yet? I've found the best way to get top class horses is to buy a whole bunch of cheap mares and breed them to the best, ie. most expensive, game stallions that you can. Gradually, over the seasons, you will build up a stable full of multiple group I winners by breeding down from the best animals you get from those breedings.
Don't know if this is the sort of advice you were after
Good luck mate.