To quote Paul Draper, who was quoting Max Maven, "Read everything."
The thing about mentalism is that it's not "tricks" per se. It's more accurately described as employing systems. The more systems you know and are proficient in, the more you can adjust your performance as needed on the fly.
You can find sources that focus on psychological forces specifically (Peter Turner did an eBook last year, for example), but unless you understand how and why they work and are able to adjust them to fit your persona and style, they won't work consistently. Even then, you won't hit 100%. Even Derren Brown misses with psychological forces sometimes.
You can't think about this like it's a magic trick. In magic if you do X, Y, and Z moves/sleights in the right order, you will succeed. In psychological forces, you may not succeed even if you do everything "right".
So if you want to start using psych forces, I'd say first study the foundation information - Fundamentals by Bob Cassidy, 13 Steps by Corinda, then look for stuff by Atlas Brookings, Peter Turner, Ross Parker - guys like that. Just understand this is something you're going to have to practice with real audiences for some time before you gain consistency.