There are several methods...

1. Purchase pre-made popper bodies like these...
http://www.jannsnetcraft.com/bug-popper-bodies/

2. Make your own from balsa wood. Buy a square stick from the hobby store, like a 1/2" X 1/2" X 36" balsa stick. Using 100 grit sand paper round over the edges until you have a round dowel of the right size. Using a pencil sharpener (or the sand paper) point one end of the dowel. Cut the point off at an angle. A round sanding bit in a dremel tool can be used to form a cupped face, if desired. Using a small hacksaw blade make a slot in the bottom for attaching to a thread wrapped hook with super glue or epoxy. Paint with spray paint or brush on acrylic paint. A sharp piece of brass tube can also be used to punch dowels from a balsa wood block, just be sure to cut in the same direction as the grain. Balsa takes paint well and is very easy to shape with sand paper.

3. Make your own from foam sandels. Use a small section of brass tube to punch plugs from foan flip-flop sandles. The brass tubes are available at the hobby store, they come in many sizes 1/4" to 1/2".
You can sharpen the end with some silicone carbide (wet/dry) sand paper or a metal file. The foam cuts very easy, so it's not much work to cut them by hand. Use a wood dowel of slightlty smaller size to push out the finished plug. If you have access to a drill press or power drill you can chuck the brass tube in it and punch lots of plugs fast. When using a drill press I like to set the depth to cut about a 1/16" short of the bottom edge, this way the plugs stay in the foam block and I don't have to stop the drill to to get then out. I later release all the plugs by slicing off the extra 1/16" inch with a sharp razor blade. Foam plugs can be further shaped with sand paper. Some folks make a mini-lathe out of a dremel tool and turn them to final shape (do a search on 'dremel bug') Foam comes in many colors and thicknesses, you can laminate various colors together with contact cement for a stripped effect. Attach the finished bodies by making a small cut on the bottom surface, glue to a thread wrapped hook with super glue.

4. Small corks can be purchased at most hardware stores or can be cut from old wine corks and shaped with sand paper. I don't like cork because of the extra work needed to fill the voids before painting. I prefer to use balsa wood.

5. Larger hardware stores sell 'foam backer rod' to be used to fill large gaps prior to caulking. These are foam rods 3/8" to 1/2" in dia. made of a soft foam. These can be cut with a beveled end and glued right on the hook with super glue. They don't take paint very well.

6. Learn to spin deer hair. Spun deer hair bodies make excellent poppers which are light, durable and easy to cast.