Right now it's as simple as just finding them lying around the ground. It may seem too easy, but with fairly lengthy maps and the Fog of War it becomes more about how much you want to run around and explore (with monsters at every turn, also behind the Fog!). Then again, that's one of the things that I'm looking for in the Alpha test; to check and make sure they're not too easy/hard to find.
All monsters also have a few specific materials that can be stolen should you wish to cherry-pick a few here and there, and there are shops that sell them at exorbitant prices.