Yes it's hard to get a level 6 trainer. But that's the point, if it was cheap and easy everyone would have a high level trainer and there wouldn't be any strategy as to what level trainer to buy.
Level 6 trainers are mainly for training NT/U21 prospects (or a similar elite player), and even then a slightly lower level trainer can also produce good results. For the majority of managers a level 4 trainer is considered the best option and very affordable.