There are a wide range of sugar-free products for flavoring water. Most of these are artificially sweetened.
They come in liquid form, where you can choose how many drops you add to your water and thereby control how strong the flavor is. Brand names include Mio and Sweet Drops; you can find many more by searching for "water flavoring drops." These products tend to be quite concentrated, so a small bottle of flavoring will do for many drinks of water. So even if you can't find them at a store near you, it's quite practical to order online and have them shipped.
They also come in powder form. Some powders are individually packaged, which is convenient to carry with you and use on the go, but allows you less control over the flavor concentration. They usually come in long, skinny packets that easily fit into the mouth of a water bottle, so you can pour in the powder without spilling it. Others come in bulk form, where you measure them out with a scoop or spoon. That lets you control the concentration, but it's not as portable and can be a bit messy. Brand names include Crystal Light and Gatorade Zero, and you can find many more by searching for "sugar free water flavoring powder."
Both types of product are, in my experience, widely available. As in, basically every gas station and convenience store has a few, and most grocery stores have a shelf with upwards of 30 flavors in several different brands.
You can also get sugar free flavored syrups, like the kind they use at fancy coffee shops. These tend to be quite sweet, and the flavor is not as concentrated as the products that are designed for flavoring water. The bottles are large and usually glass, so you would need to use them at home or put some in a smaller bottle if you want to use them on the go. Many grocery stores and liquor stores will sell some flavored syrups, and they usually have a few sugar-free options. If you want a specific flavor you may need to buy online.
If you want an unsweetened water flavor, use a flavor extract. There's a pretty standard selection of flavor extracts available in the baking aisle of any grocery store: vanilla, almond, lemon, orange, star anise, mint, coconut, and rum. Large grocery stores and specialty baking stores have an enormous range of flavors. Some flavors taste quite natural, while others taste artificial or weird. Extracts tend to have quite concentrated flavors, so a little goes a long way, but it may be challenging to get the right amount without adding too much. You may find it helpful to dilute your extract in a small bottle, and use the diluted extract for flavoring your drinking water. If you dilute the extract below 20% alcohol, it will no longer be shelf stable, so you'll need to treat it as perishable.
Note that most extracts are made with alcohol; adding a dash of extract to your water won't get you tipsy, but it might be a good idea to ask a close friend if your breath smells like alcohol after drinking your flavored water. At least in some distant future when you can A) can ask a friend to smell your breath for you; B) go out in public without a face mask; and C) get close enough to other people that they would actually notice if your breath smelled like booze.