Amazing grace tabs are available all over the web but perhaps, as an exercise, you might be interested in learning the song by ear. A great tool to help with this is VLC - the media player that plays almost anything. Install VLC and buy the copy of Amazing Grace you'd like to learn. Then use VLC to slow down the tempo of the song.

VLC is quite amazing in that it can be used to slow down the tempo of a song without changing the pitch of the notes played. It does this by inserting empty sound into the song. This is generally not something you would hear but if you slow down a song enough, you'll be able to pick it up.

This is an incredibly effective way of learning Amazing Grace without using Amazing Grace tabs.

The first step is to try and identify the bass notes in a song. My guitar teacher taught me to do this by turning the tone knob on my guitar all the way down, turning my amp down and then trying to mimic the bassline in a song by playing the notes on the low E and A strings of my guitar. Keep trying to play along until you can get the bass line of the song. You don't have to play exactly like the bassist plays but if you can get the same notes, the rest becomes easier.

The next step is to try major and minor versions of each "note" you discovered. Try this until it sounds close to right. You might need to play a major-seventh chord instead of just a major chord but you can figure out the finer notes of each chord after you've figured out the basics.

In some instances, the chords won't sound happy (major) or sad (minor), in which case, you should try chords that don't have a major or minor third in them. Like sus2 or sus4 chords, power chords and so on.

Once you know the chords, write down the notes that make up each chord and then literally try and figure out the song by playing those notes. Often times this works but sometimes you may need to delve deeper into your major and minor scales to figure out which notes are being played. Honestly, nothing is quite as effective as slowing down the song and trying every note until you get the right ones. However, doing the chord exercise I outlined above usually means you're doing less guess-work and can figure out the song a little easier (while expanding on your musical theory as a bonus!)