Where can I get either the Linerider flash source or Freerider 2 source so I can learn how to make games like those

NOWHERE!

there is your nice short answer

now if you want to lern to make a game "like those" i would look at box2dAS3 or APE (just google them)

Also you will need adobe flex 3 (free)
One of those librarys (free)
and a understaing of EMACS script like languages and a lot of paitence
(you can also use cs3/4 to do it but this stuff is free so ill post that for relevence)