Some places will sell practice locks with pins removed, but do not buy them, they are way overpriced and if you really want to understand the mechanics of locks it will serve you well to bust one open. So go to a hardware store and pick up an inexpensive but not cheap lock, crack it open and remove some of the pins (even all but one), add/remove/reorder the pins until you are really good, and then buy more locks.
Also do keep the law in mind, when I looked it up it's illegal in most if not all states to pick locks that you do not own if you are a not a locksmith (even with the owners permission), which can include obviously the locks on your apartment and locks of friends. You could probably get away with this, but the hacker interest in learning things like lock-picking is not universally seen as benevolent, and it would be stupid to get in legal trouble for a hobby
Fresh book recommendations delivered straight to your inbox every Thursday.