I've made the same clips for my hoop house as Bit described - from black plastic pipe that was right near the PVC at our Menard's store (more or less like Lowe's or Home Depot). It needs to be just about the same inside diameter as your PVC is outside diameter. For your raised beds, though, you might want to consider using the "caterpillar tunnel" method; the plastic sheeting needs to extend out past the ends a couple of feet and is bunched up and tied to stakes pounded into the ground, and cord is used to zigzag over the tunnel to hold the plastic, which can then be pushed up on the sides for access/ventilation. Here's a link to check for a picture (and much better description than mine):
http://www.growingformarket.co...
