Just to clarify, a practice lock could either be:
a) any lock that you "practice" on.
b) a cutaway lock designed to let you see the inner working while you pick.
If you are after (b), then you can either buy one (varj is selling one on varjeal.com or you can get on from lockpickshop.com) or make one if you are skilled like that. If you are skilled like that, I won't have to tell you how to make one.
If you are after (a), use your creativity. You can buy locks retail, buy them from ebay, work out a deal with a local locksmith for scrap locks, try and pilfer locks that will be discarded at gyms, pools, etc (mostly padlocks with cut shackles, but it's something). The list is limited only by your own creative forces.
Cheers!
db