LorAnn flavorings are good - make sure to get the water soluble type. With these you can add flavor to existing flavored liquid, or buy unflavored liquid and make your own flavor from scratch.
Another source of flavor is
Capella Flavor Drops - I've tried a few and they're quite good.
With both LorAnn and Capella, some flavors are stronger (more concentrated) than others, so some experimentation is required.
I usually use an eyedropper to mix a small amount (3ml) and keep track of the drop count so I can convert that to a percentage for larger amounts.
Rick