Anyone know how to make a cheap blowgun, that wouldn't involve me buying anything?  I might have access to PVC, but I doubt it, and I don't have any bamboo.

Plastic broomstick  handle? The kind that's hollow so that you can stick a dart in. Cutting the front of a PET water bottle and usin it as a mouthpiece.would work if it fits.

Quote from Sons of benjamin on Aug 17^th, 2011, 7:48pm:

Get some frozen peas and you're all set!

PVC works decently (use 1/2 inch), BUT when it gets to appreciable length it bends. Even at just 5 ft it starts bending appreciably.  
 
I recently bought a length of Electrical conduit, but it is heavier than I thought, and I need to cut it down to 6 or 5 feet, because it too is bending (though much less than the PVC does). I've toyed with the idea of Copper pipe, that might work better.

do you have any links about creating that type of fletching/wadding? I have been disappointed with paper cones covered with packing tape.

A while back slingingrat sent me an excellent PVC blowgun.  He sent it with wooden darts with paper cone fletching.  He'd be a good one to ask.

Check YouTube under the "home made airgun" videos.   Most airguns (like the one I made) use 1/2" copper for the barrel.  One fellow showed a method of making durable dart "fletching" using plastic bottles.
 
I didn't save the video...
 
I think I can describe it, though.   You need regular plastic milk jugs.  You take a propane torch, and set it up a couple of feet away from the jug to heat the plastic.  At some point, the plastic will become clear and at that point it's quite pliable.
Then, you take an ordinary plumb-bob, and push it into the plastic, making a conical depression.  You can make several of these on each side.
 
When the plastic cools again, you can cut out these plastic cones, put a nail in one end, and measure against your barrel diameter for a snug fit.
The Video showed these things impacting !/4" plywood and going right through....
 
There are tons more designs on YouTube, most all of which would make good projectiles for mouth-blown weapons as well.

For a small blowgun take a Bic pen and strip it of the ink/tip and the end cap. Then take 1 cotton swab like a Qtip and cut it in half. The 2 halfs are your darts. Add points to the darts by gluing a sewing needle to each dart shaft with epoxy. This is what I learned my freshman year of highschool. Experiment and make it your own.