I've been looking for the TLE of the first Mars Exploration Rover (MER-A,
also know as 'Spirit') currently underway from Earth to Mars, but could not
find any. Can someone give me a hint where I can find these?

I don't know where to find the numbers, but the nasa website has
graphics showing where the three spacecraft are -
http://mars.jpl.nasa.gov/mer/mission...trightnow.html