Although there are no ACEM specific OSCE books that I can find (Im sure they are coming!), there are a heap of UK Emergency Exam style books. The OSCE’s in the UK are pretty similar it seems so using the MCEM books is likely useful
Try these Books Worth a Look!
1. MCEM Part C: 110 OSCE stations, By Kiran Somani/Nitin Jain/Nick Jenkins (Buy It Here!)
2. Self-Assessment for the MCEM Part C, By Rebecca Thorpe/Simon Chapman/Jules Blackburn (Buy It Here!)