If you can get your hands on some compost, sometimes it's available thru the local waste places or the local council, or it's often available from landscaping places, you could top up the bed with that. You want about 6 inches minimum of decent soil to get things going. Preferably more, so dig it into the existing soil if necessary.
The worms will come to a garden by themselves if the conditions are right. There's no need to buy them.