I feel like I had a somewhat similar experience to you (2 yrs CVICU experience when I took the CSC, (started in CV as a new grad so only experience I had), passed CCRN 2 months prior to taking CSC, and also wanted practice ?s/exam) so here's my advise. Other than the test plan on the AACN website which lays out for you very simply what you should know, objectives and I believe a few practice ?s, there were not any practice tests I knew of at least when I took the exam (Jan 2011). I bought and read cover to cover Cardiac Surgery Essentials for Critical Care Nursing by Sonya Hardin and Roberta Kaplow (very reasonable price on amazon
for a textbook). This book should honestly be REQUIRED reading for anyone starting in CVICU. Even with 2 years of solid experience doing high-risk hearts all day every day, I learned SO much in this book, and it is not difficult reading at all (font size reasonable, paperback, pages not huge, finished it in 3 weeks doing just casual reading hear and there). At the end of each chapter is a case study with practice ?s geared to prepare you for the CSC exam (Which they state in the book). I did all of those, read the chapters and took the test first time, no problems. I know you're in school now and have a ton of reading to do (believe me I know, I'm in CRNA school now!) but if you just get the book for the practice ?s and to skim it's well worth it.
Sorry this got long but that's my advise!