Looking for a sensor that measures heart/pulse rate
The commonly used approach is to use an infrared source and sensor pressed against the skin.  Usually the finger tip is all that is needed. In addition to pulse, many have also incorporated an Oxygen measurement feature.

The 1st sensor you linked to appears to use this approach.  But instead of buying a ready made sensor, many have made a project of building their own...

This project has been done and published many times on the Internet.  There are well documented projects such as this one from Rutgeurs.  As well as this complex one from a journal called EDN.  And this one and this one from various independent hackers.