Can you buy onion juice? I have a recipe calling for it and I've never seen it before.
If you can't find the onion juice in the grocery stores, here's a website that gives the recipe of how to make it yourself.
Look in the spice section of larger grocery sorts. I have seen one marketed under the brand name of Reeses. They also have garlic juice. By the way, I live in Indiana
In any grocery store, the spice isle/salt, pepper. Some groceries might stock it near the fresh produce as well.