I passed the ISSMP exam back in May earlier this year so I have some recent experience with preparing for this certification.
At the time I was preparing, all of the practice tests that were available were based on the old ISSMP domains which changed in May 2018 so I felt they wouldn't be a huge amount of use to help assess how ready I was to take the actual test.
I did buy one practice test app from the iPhone app store (really didn't matter which one as they were all roughly the same price and contained roughly the same set of questions from what I could see in the samples) but that was to help me with mentally and physically preparing to sit through a 125 question / 3 hour exam, but I wasn't paying too much attention to the results or using it as a learning tool.
Good luck with your studies!
