If you're in nursing school, I suggest a Littmann, they're wonderful! If you're not in nursing school and plan to actively work as a CNA, I don't think that you need to worry about stethoscopes. Most places provide them if they have you do manual bp.
I am a CNA, working home health/hospice for my local med center. They did provide me with a stethoscope but I couldn't hear a thing through it. I ended up buying an MDF for about $25. For taking vitals, it's fabulous! Plus the company provides replacement parts for life.
Another vote for a Littmann Lightweight II SE. I think allheart sells them for around $50. I paid $55 for mine in a scrub shop. My mom is an RRT and left her Master Cardiologist at home one day and had to use my Lightweight and said it was every bit as good as hers, just shorter and has a plastic rather than aluminum housing around the diaphragm. Just depends on how much change you wanna spend.
What field are you in? Is this for work or school? I bought a $12.95 sprague stethoscope and it works wonderfully (I'm a nursing student). All my instructors told us to hold off buying anything expensive until we graduated if we really needed to.