Can anyone give me advise on these ?

Can you get them from the highstreet and how much are they ? Do they work ?

i ask because my midwife unit no longer listens to baby heatbeat until after 30 weeks :\( , so thought might be useful to invest in 1 so i can hear at home myself. But do they work ?

I bought the summer infant one from tesco it was about ??29, I got mine coz I hadn't been feeling as much movement as I normally did. When i hooked it up I couldnt find a heart beat and as you can imagine I really panicked, I went to the hospital and they found heart beat, I had a scan and all was well with the baby. Turns out noone had told me my placenta is at the front and thats why I couldn't detect a heart beat. I havent tried to use it since!
Amy xxx