I rent my Doppler off eBay for 10 a month it's great and really reassuring
I always found DS's H/B, it can cause worry but it always reassured me more than worried me. My doctor always said not to use one, but I did anyway, and loved it. I suggest NOT getting them unless they're the U/S gel kind, where it's almost identical to the OB's, the ones with the headphones do not work, not until you're like 30-ish weeks.
I agree if you can't find the heart beat you'll get paniced when maybe something isn't even wrong.
You can get them online on walmart, target, ect..... But I've hear they cause way more worry than reassurance.
I bought mine online! Ebay, there's a digital one with the U/S gel that comes with it for 50-60 that I bought my last pregnancy, and will with this one, and it worked awesome!