No need to use any 3rd party here.
Just run through BCG‘s simulation to understand the format.
Otherwise, if you have practiced cases, no need to do any additional preparation. Casey is in essence a digital, written case interview (in a more rudimentary format, thus easier to pass than an actual case) - this is why it is an in-between, independent stage between CV screening and first round (as opposed to e.g. McK PSG, where the results are evaluated together with your CV and cover letter).
Expert case by