No where that I know of. I assume you are wanting to use it for killing plants. Since 20% vinegar is much stronger than food grade vinegar, it generally not available to the general public. You might try searching for it on the internet if you are really set on using it.
We do sell an organic plant killer from St Gabriel Organics called BurnOut II. I is a combination of Citric Acid and Clove Oil and will kill plants much like Round-Up does. Make sure you’re not using this product on the lawn since it kills everything it touches…weeds or grass. Make sure to read and follow the direction on the package since, organic or not, it must be used correctly to be safe.