One must obtain large quantities of negative energy, which is rare indeed. Stephen Hawking has showed that, for all wormhole solutions, negative energy is required. To keep the mouth of the wormhole open and stable, large amounts of negative energy are necessary. Negative energy created using the Casimir effect would result in a wormhole too small to travel through. There are other places, besides using the Casimir effect, that we may find negative energy:
A rapidly moving mirror
High powered laser beams
Black hole event horizon