It's located in Mali, see map. The way to get there is to sail below the whole "lobe" of which Mali is part. Then you'll find a river going straight north. I don't remember which side of the split to take further north, I seem to remember you had to take the west branch.
Just stock up on food & water so you can survive for 25 days from the port closest to that river (at a decent speed of course), that should be more than enough to find Timbuktu.