I got mine (Sonoline B) online from clinicalguard.com because of the price - Amazon was significantly more expensive. I read a lot of reviews and watched youtube videos of women using that model so I knew what to expect before I bought it.
If you are a worrier, I would advise against a Doppler - it isn't THAT easy to find a HB early on - and even now, I find that without a full bladder, I can't find it. You have to take your Doppler with a grain of salt.
that said, I love it. I try to limit my use to no more than 5-10 minutes at a time. If I can't find a HB within a few minutes, I close up shop.
Good luck!