mothercare in the UK sell a prenatal heartbeat listening monitor with earphones ( thirty pounds sterling) that you can buy so you could check mothercare here to see if they have any in stock.
I bought it when I was pregnant with my son but to be honest i could never find the heartbeat and I used to get more worried! I gave it to a friend when she was pregnant and she loved it - not because she could hear the heartbeat but when she would notice little or no movement she used to put it on and as soon as she did, her daughter would start kicking! It served it's purpose, just not the way it was intended!