If you do want to invest a little money though, I would recommend higher end padlocks like the American Lock 700 series. The 700 series is an excellent practice lock to use as an intro into higher-security padlocks, and you can start by pinning it with only 2 or 3 chambers then work your way up. (It even has some serrated key pins in it; my favorite).
Kodack wrote: I actually did put up an ad on our companies for sale page looking to buy unused locks.
Why limit yourself to unused locks?
I honestly find that practicing on used (even heavily used) locks is quite helpful. Used locks often have a very different feel to them and respond very differently to picking, so they provide a good broad range of practice.