You can hire them from a couple of places, the only one I can think of off hand is firstbeats. (google doppler) They are around $39 to $49 per month to hire and about $300 to buy.
I never used one with my previous preg as DP also thought I would panic too much if I couldn't find the HB, however after 8 weeks of persistant bleeding with Max I dont think I could worry any more.
I will definately be getting one for my next pregnancy. I don't think you can hear HB for at least 10 weeks (once bubs moves out of the pelvis) and most come with instructions so you can determine the difference between maternal blood flow and bubs.
And No, they are not harmful to the baby, although majorly addictive for the mother.