I'd be a bit skeptical about any third-party source since there's no Exam Content Outline (the way there is for PMI's "normal" certifications) against which someone outside of PMI could come up with sample questions.
Having taught the DASM and DASSM courses, I can tell you that so long as you've thoroughly reviewed the course materials (including the Study Guide) and have read section 1 (the first six chapters) of CYW thoroughly, you should be fine.
