There are many threads asking for chords to songs. Here is a strategy which may help, at least for modern songs.

First, check to see if there is a MIDI file available on the web (Full song, not just the melody line). There is a good MIDI search engine on the Music Links page at Melody Lane
But be warned, there is music played automatically at this site.

many MIDI files of folk songs can be found at Lesley Nelson's site Here

When you have got a MIDI file, you also need to buy this program (download for £12.50 UK, cheaper if you are outside Europe because no VAT) MIDI Chords
This will tell you what key it is in, and list or print all the chord names. You can also ask it to transpose the chords to any other key.