I don't think trainers are necessary, but maybe they do help. I wouldn't know. I don't own any. You want to do the pass with a deck of cards, so I never understood why a person would practice with anything else. Just use cards.
The Classic Pass is an easy move to do, but a hard move to do correctly. I would highly recommend purchasing the Classic Pass download from either Jason England, Alex Pandrea or Xavier Spade. Learning it from a book or from youtube will only make you develop bad habits if you continuously practice it wrong.
Many people on here will tell you that you don't need a perfect pass since you want to manage and misdirect your audience when you do the actual move. Eh. I feel like posts like that almost encourage you to either not learn the pass at all (why use the pass if they're not looking, just cut the cards), or encourage you to settle with a mediocre pass. No. Learn the pass, and learn it properly. I do agree that you should try to manage your audience when doing the Classic Pass, but it also doesn't hurt to master the mechanics. You can manage the audience all you want, but even condoms are only 98 percent effective. If you really don't want a baby you'll use condoms and pull out.