It depends on where you are in the world (don't answer that !!!).
If you are in a region where sulphuric acid is controlled, such as the EU, the safest way to source it is from old car batteries. Otherwise you might find battery acid sold in the hardware store.
Acid from batteries is dirty and usually one needs to at least let it stand for a week or so to let all the impurities fall to the bottom. Then you decant off the clean acid and boil it down to evaporate water (water boils at 100 C, sulphuric acid well above 300). There are several descriptions of the process here on the forum.
Important: Observe safety procedures, when handling acids. Protect your respiratory organs and eyes! If you have battery electrolyte, you can obtain high concentrations of sulfuric acid in a simple way. This is done by evaporating the electrolyte in a sand bath. To evaporate you will need: a...
chemforum.info
For our experience, we need a battery electrolee. I bought at once a can of 5 liters with a density of 1.28 g / cm3 (which corresponds to about 36% concentration). As probably everyone knows, the electrolyte is dilute sulfuric acid. In order to obtain more concentrated acid, we need to try to...
chemforum.info
To get clean, concentrated sulphuric acid, you need to distill it. Check youtube for this, it's not exactly dfficult but can be unsafe and produces a lot of corrosive/toxic fumes.