leaky windows!

How hard is it to replace all of the window seals.....Can't seem to even get most of the windows down to replace em! Also the quater windows..I have one thats rusted out and almost came totaly off in my hand.Any ideas on what direction I should start taking to fix this. where can I get these parts?...Should I just start looking in a junk yard?..Any suggestions would be helpful,especially on the removal and instalation of the new stuff.. thanks, Chris