Good quality balsamic vinegars that have been aged and reduced by evaporation in a barrel are wonderful but expensive. If you'd rather not spend money on it you can take regular balsamic vinegar and reduce it yourself.
Take 1 cup balsamic vinegar and place it in a non-stick skillet over medium-high heat. Bring to a boil and then reduce heat to medium-low, bringing the vinegar to a simmer. Keep at a simmer for about 3 to 4 minutes until it is a little thinner than you would like it to be. It will continue to thicken after it is taken off the heat.