I use citric acid as an acidifying agent. Most commercial growers use it because it’s cheap. Food grade citric acid is available in 50 lb. bags, from chemical suppliers.
For a backyard grower, it’s also available in some grocery stores. Even if you can’t find it in grocery stores, it’s easily available on online in places like Ebay, Amazon.
As Drew mentions, I would not use strong acids like battery/sulfuric acid for sprays. Those kinds of strong acids can have unpredictable effects on the foliage.
In terms of rate, it does depend on the pH you start with and the buffer strength of the water, so it can differ.
However, I get, and appreciate, your question, that you want a ball park. I can tell you I’ve measured my water carefully, multiple times and can say that one teaspoon of citric acid per 16 gallons of water will lower the pH of my water to 5.9. One teaspoon per 24 gallons of water will bring the water to a neutral pH (7.0)
I’ve read general guidelines for farmers to acidify water. They ask the very same questions you are. Here is one from MSU. They posit that two ounces of citric acid per 100 gal of water will lower pH from 8.3 to 5.4.
Interestingly, the rate I use of one teaspoon per 16 gal of water to produce a pH of 5.9, equates to roughly one ounce per 100 gal of water. It figures to a small handful per 400 gal. tank of water, lol.
Seriously though, these numbers should give you at least a ballpark of how to acidify your water. Incidentally citric acid is a much stronger acidifying agent by weight than vinegar.