Welcome!
Since the RepRap project hasn't achieved self-replication yet, there's no good source of those parts.
If you wanted to order them today, I think your only option would be to order them from a commercial rapid prototyping company. A few people have done that, but the cost is rather high ($2-3k I think). The files you would need to send them are somewhere in the svn repository, I think. If you're still interested in doing that, I'm sure someone can point you to them.
Quite a few people are trying to build a "RepStrap" by attaching an extruder to a commercially-available (or home-built) Cartesian robot, which will set you back a couple hundred dollars. There are also a few people trying to build parts identical to Darwin's out of wood or aluminum. They're also experimenting with casting some parts from molds. If you poke around the builder blogs, you'll see a lot of different approaches.
Or, you can wait until we get the hang of replication. We should see some very affordable part kits hitting the online store shortly after that. You can buy & build all the electronics and motors now, but do keep in mind that this design isn't 100% proven and you're taking a bit of a chance.
