There are other ways to do it but basically, a hydronium ion is a hydrated hydrogen ion and a hydronium ion can be formed by hydrating a concentrated acid if it's the right acid. For example, H2SO4, if you have the 98-99% stuff, easily hydrates. Concentrated HCl, on the other hand, is only about 35% pure, so most of those hydrogen ion are already hydrated. By the way, you do NOT (repeat NOT) want to add a few drops of water to conc H2SO4. It isn't safe. Heat, spattering, boiling, etc. And conc H2SO4 is no fun if it spatters on you or in your eyes. Loss of eye sight, horrible burns, you get the picture.