The 'Parker-style' ballpoint refill is actually a standard pattern refill known as the G2, and is used by many manufacturers for their ballpoint pens. It was originally created by Parker, and is best known as their refill, but it is now an ISO standard, and is used by many different pen brands. The G2 refill is 98mm long and approx 5.8mm diameter on the main barrel. They are easily recognisable by the distinctive shape and the sculpted plastic end which is used by some pen retractor mechanisms. Most have a metal barrel, though some have plastic barrels. Metal-barrelled refills generally have a larger ink capacity than plastic versions as the barrel walls can be much thinner.
The Parker version is of excellent quality, and there are also alternatives available from manufacturers such as Pelikan, Schmidt and Schneider, which offer additional tip size and colour choices. A few interesting options worth noting:
Fun fact: the interestingly-shaped plastic part at the top of these refills was originally designed by Parker such that the mechanism would rotate the refill a quarter of a turn each time it was retracted, so the tip would get worn more evenly in use.