UWorld. Step Up to Medicine/Step Up to Step 2 have good biostats sections that are sufficient for what's tested on Step 2.
The questions you'll get are mostly all about the same things: sensitivity, specificity, PPV, NPV, attributable risk, absolute risk reduction, relative risk, odds ratio, number needed to treat/harm, types of studies and their benefits/downfalls (cohort, experiment, case-control, cross-sectional)... they just like to mix them up and ask it in different ways so just know how to calculate each one and what they each mean.
Once you get the hang of them they can become the easiest questions on the test. Be careful though they present the graphs/tables in different ways in different questions so remember to look at the labels and make sure you're calculating the right thing.