Hi SJ, These guys are called 'Mills' pins or Roll pins.  You can use a
1/4-20 x4" bolt to drive out the other one and just take it to a serious
hardware shop and get another (or two)  The trick is to use a hammer to
drive out the mills pin using the bolt as a driver.  Leave the bolt in the
armrest, you'll use it to put the old pin back in by tapping it gently into
the hole where the threaded end of the bolt resides.  Sounds more
complicated than it really is though.  When you have the two new pins, put
one in the armrest from which you got the sample.  Then tap in in until it's
almost completely installed and then remove the 1/4-20 and use it to seat
the pin the rest of the way.  Then use the bolt to align the other armrest
and tap it through so you have a guide for the second new pin.  Keep the old
pin as a spare.  Keep the bolt as a tool.

Al