If its the model supplied without the heart rate monitor, you can use any ANT+ compatible heart rate strap. The obvious choice would be Garmin's own strap.
Have a look on the amazon web site to see the associated HRM strap:
http://www.amazon.co.uk/Garmin-Forerunner-Enabled-Unisex-Sports/dp/B003EG83LK/ref=sr_1_1?ie=UTF8&qid=1322842532&sr=8-1
You can probably get a better deal elsewhere, but this shows what you are looking for.