I am preparing for the PST as well and I have reserached extensively all available resources. The three official tests are obviously the best practice, but they are just 3, and I really wanted to give the best on that day. Among all the sources, I found that the best ones are on myconsultingcoach.com , mainly for three reasons:
- They have a guide that teaches you tips and tricks and will probably make you save a bit of time in the exam (that you can use for other questions, since there's not much of it ;)
- They break down all the questions in 5 types and provide you with a tracking of your performance by area, so that you are able to focus your preparation on the areas where you are weaker.
- Their tests are VERY CHALLENGING, probably even more than the real one, but they definitely train you to think in the right way, and at the end of the day setting the bar high is not a bad idea, especially given the amount of people who recently took the test and claimed it was harder than expected.
Hope it helps!